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AGRICULTURAL  BACTERIOLOGY 


CONN 


BY  THE  SAME  AUTHOR 

BACTERIA   IN   MILK 

AND  ITS  PRODUCTS 

12MO.  306  PAGES 

CLOTH,  SI. 25   net 

AGRICULTURAL 

BACTERIOLOGY 

A  STUDY  OF  THE 

RELATION    OF   GERM    LIFE   TO   THE   FARM 

WITH 

.  LABORATORY  EXPERIMENTS  FOR  STUDENTS 


Microorganisms   of   Soil,   Fertilizers,   Sewage,  Water, 
Dairy  Products,  Miscellaneous  Farm  Products 

AND 

OF   Diseases   of  Animals  and   Plants 


BY 

H.  W.  CONN,  Ph.  D. 

Professor  of  Biology  at  Wesley  an  University 

AUTHOR  OP  **  THE   STORY  OF  GBRM  LIPB,"  "  BACTERIA   IN  MILK  AND  ITS  PRODUCTS,"  "  PRACTICAL 
DAIRY  BACTERIOLOGY,"  "  BACTERIA,  YEASTS,  AND  MOLDS  IN  THE  HOMB,"   ETC. 

UWVEMITY  OF  CAUFORNIA 

LIBfjARY 

CX)LLEGE  OF  AGRICULTURE 
DAVIS 

SECOND  EDITION^  RI^VISED  ANX^eNLARGED 

WITH  64  TLLUSTRATIpN§   , 


PHILADELPHIA 

P.   BLAKISTON'S  SON  &  CO. 

1012   WALNUT  STREET 
1909 


'U^,<^,  "llroJr^^  "      ''^^ 


Copyright,  1909,  by  P.  Blakiston's  Son  &  Co 


Printed  by 

The  Maple  Press 

York,  Pa. 


PREFACE  TO  SECOND  EDITION 


Since  the  publication  of  the  first  edition  of  this  work  advance 
along  all  lines  of  bacteriology  has  been  very  rapid.  Scarcely  a 
phase  of  the  relation  of  bacteria  to  agriculture  has  failed  to  receive 
substantial  contributions.  Much  new  information  has  been 
obtained,  the  relative  importance  of  different  subjects  has  been 
changed,  and  in  a  few  points  our  previous  conclusions  have  been 
corrected.  These  changes  have  been  so  considerable  that  in 
preparing  this  second  edition  it  has  been  found  necessary  to  rewrite 
the  whole  book  in  order  to  bring  it  up  to  the  times.  The  subject  has 
grown  so  large  that  it  is  difficult  to  include  within  the  limits  of  one 
volume  even  the  fundamental  facts  of  the  rapidly  growing  science, 
and  many  subjects  of  importance  have  been  treated  very  briefly. 

The  growing  recognition  of  the  importance  of  the  subject  to 
students  of  agriculture  has  caused  agricultural  schools  and  col- 
leges to  give  to  it  an  increasing  amount  of  attention.  For  this 
reason  this  edition  has  been  planned  with  special  reference  to  its 
use  by  classes;  and  some  changes  in  method  of  presentation  have 
been  adopted  in  order  to  make  it  more  useful  to  students.  For 
the  same  reason  there  has  been  added  a  somewhat  extended  set  of 
experiments  for  elementary  laboratory  work.  These  laboratory 
directions  are  far  from  exhaustive  and  are  designed  simply  to  intro- 
duce the  student  to  the  methods  of  bacteriological  work. 

The  close  relation  of  the  functions  of  the  higher  fungi  to  the 
functions  of  bacteria  has  come  to  be  fully  realized  to-day  and  has 
made  it  necessary  to  include  more  extended  references  to  the  higher 
fungi  in  this  review.     The  only  other  important  addition  in  this 

035444- 

42.744 


VI  PREFACE   TO    SECOND   EDITION. 

edition  is  a  considerable  extension  of  the  treatment  of  bacterial 
diseases  of  plants  that  have  received  so  much  investigation  in  the 
last  ten  years.  With  these  additions  it  is  hoped  that  the  present 
edition  will  be  a  fair  summary  of  our  present  information  upon  the 
relation  of  germ  life  to  agriculture. 

H.  W.  C. 

MiDDLETOWN,  CONN. 

June  15,  1909. 


PREFACE  TO  FIRST  EDITION 


To  set  any  exact  limits  to  Agricultural  Bacteriology  is  difficult. 
Primarily  the  subject  includes  only  phenomena  produced  by 
bacteria,  and  phenomena  that  especially  affect  agriculture.  But 
some  agricultural  processes  are  so  closely  bound  with  other  industrial 
phenomena  that  they  cannot  be  separated.  Agriculture  grades 
by  imperceptible  degrees  into  numerous  secondary  industries. 
Quite  a  number  of  the  phenomena  which  will  be  considered  in  these 
pages  have  a  closer  relation  to  these  secondary  industries  than 
they  do  to  agriculture  proper,  but  nevertheless  they  do  have  at 
least  an  incidental  relation  to  the  farm  and  must,  therefore,  be 
included  in  a  discussion  of  Agricultural  Bacteriology. 

It  has,  moreover,  in  recent  years,  been  a  growing  conviction  that 
a  considerable  number  of  phenomena,  hitherto  attributed  to  bacteria, 
are  directly  due  to  a  class  of  chemical  ferments  called  enzymes. 
These  enzymes  are  sometimes  produced  by  bacteria,  but  in  other 
cases  by  organisms  totally  unrelated  to  bacteria.  When  the  latter 
is  the  case  the  fermentations  produced  by  them  have,  of  course, 
nothing  to  do  with  bacteriology  proper.  But  we  do  not  know 
as  yet  how  commonly  these  enzymes,  or  chemical  ferments,  are 
concerned  in  agricultural  processes,  and  even  where  they  do  occur 
it  is  found  that,  in  some  cases,  they  are  intimately  associated  with 
true  bacteriological  action.  It  is  impossible  to  separate  chemical 
from  biological  fermentations  by  a  hard  and  sharp  line,  nor  can  we 
tell  to-day  how  far  both  of  them  may  be  concerned  in  any  particular 
type  of  fermentations.  In  the  following  pages,  therefore,  it  will 
be  necessary  to  consider,  to  a  certain  extent,  both  types  of  fermenta- 

vii 


Vlll  PREFACE    TO    FIRST    EDITION.- 

tion.  While  both  must  be  described  and  discussed,  the  bacterio- 
logical fermentations  will  demand  most  of  our  attention.  For  all  of 
these  reasons  the  limits  which  we  shall  draw  to  the  subject  of 
agricultural  bacteriology  are  somewhat  arbitrary,  and  some  of  the 
topics  here  considered  may  not  be  regarded  as  belonging  strictly 
to  the  subject.  All  of  them,  however,  have  at  least  an  incidental 
relation  to  the  farmer  and  his  industry. 

H.  W.  C. 

MiDDLETOWN,  CONN. 


TABLE  OF  CONTENTS. 


PART  I. 

General  Nature  of  Microorganisms  and  Their  Activities. 

CHAPTER  PAGE 

I. — The  General  Characters  of  Microorganisms .    ...  i 

II. — ^Fermentation,  Putrefaction,  and  Decay 22 

PART  II. 

Bacteria  in  Soil  and  Water. 

III. — Nature's  Food  Supply.     The  Carbon  Cycle  .    ...  37 

IV. — Decomposition  of  Nitrogenous  Compounds   ....  47 

V. — Nitrification  and  Denitrification 57 

VI. — The  Manure  Heap  and  Sewage 69 

VII. — Reclaiming  Lost  Nitrogen 90 

VIII. — Bacteria  and  Soil  Minerals in 

IX. — Some  Practical  Lessons  from  Soil  Bacteriology.    .    .  118 

X. — Bacteria  in  Water 127 

PART  III. 

Bacteria  in  Dairy  Products. 

XL — Bacteria  in  Milk 137 

XIL— Control  of  the  Milk  Supply  .    . 165 

XIII. — Bacteria  in  Butter-making 181 

XIV. — Bacteria  and  other  Microorganisms  in  Cheese-making  195 

ix 


X  TABLE    OF    CONTENTS. 

PART  IV. 

Relation  of  Microorganisms  to  Miscellaneous 
Farm  Products. 

CHAPTER  PAGE 

XV. — Alcohol,   Vinegar,   Sauer    Kraut,  Tobacco,    Silage, 

Flax 211 

XVI. — Preservation  of  Food  Products 235 

PART  V. 

Parasitic  Bacteria. 

XVII. — Resistance  Against  Parasitic  Bacteria 251 

XVIII.— Tuberculosis 261 

XIX. — Other  Germ  Diseases 280 

XX. — The  Parasitic  Diseases  of  Plants 293 

PART  VI. 

Appendix. 

XXI. — Laboratory  Work  and  Disinfection 307 

Index 327 


PART  I. 

GENERAL    NATURE    OF    MICROORGANISMS    AND 
THEIR  ACTIVITIES.  . 


CHAPTER  I. 
THE  GENERAL  CHARACTERS  OF  MICROORGANISMS. 

MICROORGANISMS  AND  FARM  LIFE. 

The  successful  farmer  of  to-morrow  will  be  the  one  who  most 
skillfully  regulates  the  growth  of  microorganisms.  Though  he 
may  not  be  conscious  of  it,  much  of  the  work  that  the  farmer  is 
carrying  on  even  now  is,  as  we  shall  see,  really  directed  toward  the 
control  of  germ  life.  In  the  consideration  of  the  microorganisms 
related  to  farm  life  we  are  concerned  with  three  different  types  of 
plants :  Bacteria,  Yeasts  and  Higher  Fungi.  Although  the  latter  are 
plants  of  considerable  size,  and  hence  hardly  microorganisms,  in  many 
respects  they  are  related  to  the  microscopic  bacteria  and  yeasts, 
and  the  functions  of  all  three  in  farm  life  are  so  similar  that  all 
must  properly  be  considered  together.  The  molds,  being  plants 
of  considerable  size,  have  been  known  a  long  time,  although  only 
recently  has  their  relation  to  nature's  processes  been  understood. 
Yeasts  were  used,  under  the  name  of  leaven,  far  back  in  history; 
but  it  was  not  till  1680  that  the  Dutch  microscopist,  Leeuwenhoek, 
showed  with  his  microscope  that  yeast  consists  of  minute  globules; 
and  it  was  150  years  later  when  Schwann  and  Caignard-Latour 
proved  yeast  to  be  a  living  plant.  Leeuwenhoek  was  also  first  to 
see  bacteria,  and  studied  them  as  early  as  1695.     His  descriptions, 

I 


2  THE    GENERAL   CfiA.RACTERS    OF   MICROORGANISMS. 

corisiSefing  the  fad  f Rat  Tie* had  only  simple  lenses  to  work  with, 
were  remarkably  correct.  Even  his  suggestions  concerning  their 
nature  sound  quite  modern  and  were  certainly  superior  to  much  of 
the  speculation  that  followed.  He  intimated  that  they  might  be  the 
cause  of  disease.  But  for  150  years  after  Leeuwenhoek,  although 
the  microscope  became  a  familiar  plaything,  it  was  hardly  thought 
that  these  minute  organisms  offered  a  subject  for  serious  study. 
For  a  century  they  were  simply  objects  of  speculation,  and  many 
were  the  exclamations  which  they  excited  as  to  the  wonders  of  nature, 
with  here  and  there  a  suggestion  as  to  their  possible  importance  in 
producing  certain  natural  phenomena. 

Relation  to  Disease. — Not  until  toward  the  middle  of  the 
nineteenth  century  was  it  conceived  that  the  microscopic  organisms, 
at  first  grouped  together  under  the  general  head  of  animalculce, 
could  have  more  than  scientific  import.  At  that  time  there  began 
to  appear  suggestions  as  to  their  possible  relations  to  certain  diseases, 
and  almost  simultaneously  they  were  thought  of  as  csiusmg  fermen- 
tations. Even  before  it  was  known  what  yeast  was,  it  was  recognized 
as  in  some  way  associated  with  alcoholic  fermentation;  but  not  till 
about  1838  was  it  clearly  proved  that  yeast  plants  are  the  cause  of 
the  fermentation  of  sugar.  The  development  of  a  knowledge  of 
bacteria  fallowed  a  little  later.  One  of  the  first  real  contributions 
to  a  knowledge  of  their  significance  was  the  demonstration  in  1840, 
of  the  fact  that  certain  microscopic  organisms  cause  blue  milk. 
Thus,  at  the  very  beginning  of  the  modern  study  of  bacteria,  they 
were  associated  with  peculiar  agricultural  phenomena,  an  interesting 
fact  when  we  notice  that,  in  the  next  quarter  of  a  century  or  more, 
the  chief  investigations,  and  all  the  interest  in  them,  centered  around 
the  question  of  their  agency  in  producing  disease.  Bacteria  are 
still  suffering  in  reputation  from  the  fact  that,  for  thirty  years,  they 
were  studied  by  microscopists  chiefly  from  the  standpoint  of  their 
agency  in  the  production  of  disease.  It  was  quite  early  suggested, 
and  soon  demonstrated,  that  these  little  plants  have  the  power 
of  producing  certain  dreaded  diseases,  and  the  reputation  which 
they  thus  obtained  still  clings  to  them.  The  very  word  bacteria,  or 
germs,  has  become,  in  the  minds  of  some,  almost  synonymous  with 


MICROORGANISMS   AND    FARM   LIFE.  3 

disease.  Their  relatiou  to  the  medical  profession  was  soon 
recognized,  and  more  or  less  extended  courses  in  bacteriology  have 
rapidly  made  their  appearance  in  medical  schools.  Health  boards 
and  sanitary  officers  have  recognized  that  their  primary  duty  is 
to  deal  with  bacteria;  and  most  of  the  regulations  for  the  preserva- 
tion of  the  public  health  have  been  directed  toward  the  destruction 
or  control  of  these  organisms. 

As  more  information  has  accumulated  during  the  last  twenty 
years  or  so,  it  has  become  evident  that  microorganisms,  including 
bacteria,  do  not  deserve  all  the  ill  repute  that  they  have  acquired. 
It  has  been  learned  that  there  are  hundreds  and  even  thousands  of 
kinds  of  bacteria,  and  that,  while  certain  species  are  the  cause  of 
disease,  others  are  harmless,  some  are  beneficial  in  the  body,  and 
many  perform  functions  of  the  highest  significance  and  value. 
Although  the  disease  side  of  the  bacteria  story  was  the  first  to  be 
studied,  it  is  only  a  small  part  of  the  subject.  Among  the  many 
hundred  kinds  of  bacteria  known,  only  a  few,  less  than  two  score, 
are  as  yet  definitely  known  to  have  any  power  of  causing  disease 
in  man.  As  bacteriologists  have  widened  their  views  and  looked 
outside  of  the  human  body,  they  have  found  that  these  organ- 
isms are  not  only  excessively  abundant  in  nature,  but  have 
relations  to  the  phenomena  of  living  things  which  were  wholly 
unsuspected.  Within  the  last  twenty  years  a  larger  and  larger 
amount  of  attention  has  been  directed  to  the  part  played  in  nature 
by  microorganisms  which  are  never  parasitic  and  have  no  relations 
to  human  disease.  As  a  result  there  has  developed  a  new  branch  of 
bacteriology  which  deals  with  phenomena  wholly  separate  from 
disease. 

Relation  to  Agriculture. — In  particular  it  has  been  shown 
that  bacteria  are  related  to  agriculture.  Not  only  is  it  true  that 
they  are  the  cause  of  certain  animal  and  plant  diseases  with  which 
the  farmer  has  to  contend,  but  it  is  becoming  manifest  that  they  are 
intimately  associated  with  many  normal  processes  which  are  going 
on  in  the  soil,  water,  and  elsewhere,  and  that  they  are  fundamental 
to  the  processes  of  agriculture. 

The  agricultural  side  of  bacteriology  is,  if  possible,  more  impor- 


4  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

tant  than  the  pathological  side.  If  the  medical  student  needs  to 
know  something  of  these  organisms  and  their  relations  to  disease, 
even  more  does  the  agriculturist  need  to  understand  their  relations 
to  his  industry.  These  microorganisms  play  such  a  fundamental 
part  in  the  processes  of  nature  that  the  life  phenomena  of  animals 
and  plants  are  inextricably  bound  up  in  the  functions  of  bacteria, 
and  without  them  life  processes  must  soon  cease.  The  physician, 
in  the  curing  of  disease,  gains  a  certain  advantage  from  his  knowl- 
edge of  bacteria;  but  the  farmer  is  obliged  to  make  use  of  these 
agents  in  a  large  number  of  his  farming  processes;  hence,  it  is  a 
matter  of  necessity  that  the  agriculturist  of  the  future  should  have 
a  practical  knowledge  of  the  general  phases  of  bacteriology.  The 
solution  of  the  most  vital  agricultural  problems,  like  that  of  continued 
soil  fertility,  involves  bacteriology.  From  beginning  to  end  the 
occupations  of  the  farmer  are  concerned  in  the  attempt  to  obtain  the 
aid  of  these  microorganisms  where  they  may  be  of  advantage,  and 
to  prevent  their  action  in  places  where  they  would  be  a  detriment. 
The  farm  cannot  be  properly  tilled  unless  the  farmer  has,  in  addi- 
tion to  his  seed  crop  and  cattle,  a  stock  of  the  proper  kind  of  bacteria 
to  aid  him  in  preparing  the  soil  and  in  curing  the  crops.  Farming 
without  the  aid  of  bacteria  would  he  an  impossibility,  for  the  soil  would 
yield  no  crops. 

The  relation  of  microorganisms  to  farm  life  is  one  of  the  most 
recent  branches  of  science.  Scarce  twenty  years  have  elapsed  since 
the  first  steps  in  this  direction  were  taken,  and  some  of  our  scientists, 
who  are  still  young,  have  seen  practically  the  whole  development  of 
the  subject  from  its  starting-point  in  the  early  eighties.  With  a 
science  as  young  as  this,  it  is  inevitable  that  many  questions  remain 
unsolved.  Scientific  discovery  usually  precedes  any  practical 
application,  and  in  these  early  years  of  the  development  of  agricul- 
tural bacteriology  we  must  expect  to  find  the  theoretical  side  of  the 
subject  proceeding  rapidly,  while  the  application  of  the  facts  to 
farm  methods  lags  behind  and  is,  in  many  respects,  hesitating, 
tentative,  or  even  unsatisfactory.  Nevertheless,  the  discover- 
ies made  have  already  revolutionized  agricultural  processes. 
Changes  in  agricultural  methods,  due  to  bacteriology,  have  been 


WHAT    ARE    MICROORGANISMS.''  5 

largely  adopted  all  over  the  world;  but  they  have  generally  been 
adopted  by  farmers  in  ignorance  that  they  are  benefiting  from 
bacteriological  research.  That  these  practical  applications  of 
bacteriology  to  agricultural  processes  will  increase  with  the  next 
few  years  is  certain.  Successful  agriculture  of  the  future  is  in- 
dissolubly  bound  up  in  the  problem  of  the  proper  handling  of 
microorganisms.  We  have  reached  a  point  where  every  advanced 
farmer,  who  wishes  to  put  himself  into  a  proper  condition  to  make 
the  best  use  of  the  means  at  his  disposal  and  to  profit  by  discoveries 
as  they  are  made,  must  at  least  have  a  general  knowledge  of  the 
fundamental  factors  of  bacteriology  as  they  are  related  to  agriculture. 

WHAT  ARE  MICROORGANISMS? 

In  studying  the  relation  of  germ  life  to  the  farm  we  are  concerned 
with  a  class  of  phenomena  called  fermentation,  putrefaction,  decay, 
decomposition,  and  the  like  (see  Chapter  II).  These  phenomena 
are  all  caused  by  living  bodies  that  are  frequently  called  microorgan- 
isms. This  term  strictly  means  animals  or  plants  of  microscopic 
size.  But  this  conception  of  the  term  is  at  once  too  narrow  and  too 
broad  to  cover  the  organisms  we  are  to  study.  Some  microscopic 
organisms  have  no  particular  relation  to  the  classes  of  phenomena 
which  we  are  considering.  A  great  host  of  microscopic,  green 
water-plants  and  also  many  microscopic  animals  have  nothing  to  do 
with  our  subject,  though  they  might  properly  be  called  micro- 
organisms. On  the  other  hand,  some  plants  of  large  size,  like  molds 
and  toadstools,  have  a  part  to  play  in  producing  the  decomposition 
of  organic  structures  very  similar  to  that  played  by  bacteria.  These 
cannot  properly  be  called  microorganisms,  but  nevertheless  they 
must  be  included  with  the  study  of  bacteria  and  yeasts,  since  they 
perform  similar  or  closely  allied  functions  in  nature.  A  better  term 
to  cover  the  organisms  which  we  must  study  might  be  the  rather 
broad  term  of  Fungi,  for  all  of  the  organisms  with  which  we  are 
concerned  belong  to  this  general  class  of  plants.  But  this  term  is 
also  unsatisfactory,  since  it  fails  to  convey  the  idea  that  the  organisms 
are  largely  microscopic.     To  most  people  the  term  fungus  gives  at 


6  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

once  the  impression  of  a  large  plant;  and  we  are  chiefly  concerned 
with  microscopic  forms.  We  shall,  therefore,  still  use  the  term 
microorganism,  although  some  of  the  plants  that  we  shall  refer  to 
are  not  microscopic.  In  this  discussion  we  are  concerned  chiefly 
with  Bacteria,  secondarily  with  Yeasts,  and  to  a  less  degree  with 
the  Higher  Fungi. 

The  Fungi. — All  of  the  plants  with  which  we  are  here  concerned 
belong  to  the  class  that  botanists  call  Fungi.  There  is  one  charac- 
teristic common  to  all  Fungi — they  all  lack  green  coloring  matter. 
This  green  coloring  material  in  ordinary  plants  makes  it  possible  for 
them  to  live  upon  the  mineral  ingredients  in  the  soil;  and  green 
plants  only  can  be  thus  nourished.  The  colorless  plants  are 
unable  to  obtain  nourishment  from  the  mineral  world.  The  Fungi, 
since  they  are  all  colorless,  must  live  upon  food  furnished  them  by 
other  plants  or  animals.  It  is  this  fact  that  gives  them  their  signifi- 
cance in  nature,  and  explains  their  important  relations  to  farm  life. 
Fungi  are  very  abundant  everywhere,  and  there  are  thousands  of 
different  kinds.  For  the  purpose  of  our  study,  we  may  recognize 
three  groups: 

I.  Higher  Fungi. — Under  this  general  name  we  will  include  a 
large  number  of  colorless  plants,  mostly  of  large  size.  It  includes 
such  plants  as  molds,  mushrooms,  toadstools,  tree  fungi,  and  hosts  of 
others  less  commonly  known.  Some  of  them  are  of  great  importance 
in  farm  life,  especially  as  agents  in  bringing  about  the  decomposition 
of  vegetable  matter,  so  that  it  may  be  incorporated  into  the  soil  to  be 
used  again;  here  they  play  a  part  secondary  only  to  bacteria.  They 
are  of  endless  variety,  and  it  would  be  manifestly  impossible  here  to 
attempt  any  consideration  of  their  classification.  One  point  con- 
cerning them  must  be  understood.  In  all  the  higher  Fungi  with 
which  we  are  concerned  the  body  of  the  plant  consists  of  a  mass  of 
delicate  threads  which  grow  into  a  dense,  usually  white  mass  (Fig.  i). 
Sometimes  the  threads  are  large  enough  to  be  seen  easily  and  some- 
times they  are  so  delicate  that  a  microscope  is  required  to  see  the 
individual  threads,  though  the  mass  of  threads  may  be  of  consider- 
able size.  The  mass  of  threads  grows  on  the  surface  or  in  the 
substance  upon  which  the  plant  is  feeding.     This  thread  is  able 


WHAT   ARE    MICROORGANISMS?  7 

in  many  cases,  by  growing,  to  force  its  way  into  the  solid  mass 
of  hard  substances,  like  wood,  and  to  push  itself  between  the  wood 
fibers.  Thus  it  is  a  primary  agent  in  effecting  the  destruction  of 
wood.     Such  a  mass  of  branching  threads  is  called  a  mycelium,  and 


Fig,  I, — One   of  the  higher  fungi,  the   common  bread  mold,  Penicillium  glaucum. 
a,  the  whole  plant;  b,  one  of  the  spore-bearing  branches  more  highly  magnified. 

is  found  in  all  the  Fungi  of  this  class.  So  far  as  the  mycelium  is 
concerned,  most  of  these  plants  are  much  alike.  But  the  different 
species  have  many  different  methods  of  reproduction,  and  it  is 
chiefly  upon  their  reproductive  bodies  that  botanists  rely  to  distin- 


FiG.  2. — Mucor,  a  common  mold, 
showing  mycelium  and  spore  for- 
mation. 


Fig.  3. — Aspergillus,  a  common  mold,  show- 
ing mycelium  and  spore  formation. 


guish  the  different  species.  After  the  mycelium  has  grown  for  a  little 
time,  it  commonly  sends  up  into  the  air  small  or  large  branches  that 
produce  spores,  or  reproductive  bodies.  The  method  of  spore 
production  differs  sufficiently  in  the  different  fungi   to  make   it 


8  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

possible  to  classify  them.  Frequently  only  the  spore-producing  part 
of  the  plant  is  seen,  and  it  may  be  the  only  part  known,  except 
to  botanists.  For  example,  the  toad  stool  is  only  the  reproducing 
portion  of  a  fungus;  it  has  a  mycelium  wholly  under  ground  or 
buried  within  the  hard  mass  of  the  trunk  of  a  tree.  It  is  the  myce 
lium,  however,  that  does  the  work  for  which  these  fungi  are  re 
sponsible,  and  not  the  spore-producing  part  that  we  see.  Figs,  i  to  3 
show  the  general  appearance  of  some  of  these  fungi  and  their 
methods  of  forming  spores.  With  these  methods  of  reproduction 
and  classification  we  are  not  concerned  in  this  work,  and  only  such 
types  as  are  related  to  our  subject  will  be  mentioned  later  in  their 
proper  places. 

2.  Saccharomyces  (Yeasts,  Budding  Fungi) . — These  immensely 
important  plants  are  all  microscopic  in  size.  While  varying 
somewhat,  an  average  size  is,  about  i  /4000  of  an  inch  in  diameter. 
They  are  usually  spherical  or  oval  in  shape,  though  sometimes 
slightly  elongated  (Fig.  4,  a).  They  form  no  mycelium  and  cannot 
force  their  way  into  hard  substances.  Their 
chief  characteristic  is  their  method  of  reproduc- 
tion by  a  process  called  budding.  There  appears 
on  the  side  of  the  yeast  cell  a  minute  bud,  which 
continues  to  increase  in  size  until  it  becomes  as 
large  as  the  cell  from  which  it  has  grown.  Then 
Fig.  4.— Yeast  the  two  cells  may  break  apart  at  once;  or  each 
me^hVd'if'g^rowth  by  ^^Y  ^^  ^^^^  producc  buds  before  they  separate, 
budding,    a,  single    In  either  casc,   two  or  more  cells  are  produced 

cells;  &,  budding  cells,      r  i  1       i  1  i       i 

from  the  one,  and  although  they  may  remam 
attached  so  as  to  form  irregular  masses  of  several  cells,  (Fig.  4,  b), 
each  cell  is  really  complete  in  itself.  Eventually  they  break  apart. 
This  budding  takes  place  rapidly,  though  not  so  rapidly  as  the 
division  of  bacteria,  which  will  be  mentioned  later. 

A  second  important  character  of  yeasts  is  the  nature  of  the 
fermentation  they  produce.  They  have  an  action  especially  upon 
sugars,  which  they  break  up  into  carbonic  acid  and  alcohol.  This 
action  makes  them  play  a  large  part  in  nature's  processes,  quite 
distinct  from  that  of  bacteria. 


GENERAL    CHARACTERS    OF   BACTERIA.  9 

Any  further  classiiication  of  yeasts  is  quite  unnecessary  for  our 
purposes. 

3.  Schizomycetes  {Fission  Fungi,  or  Bacteria). — This  group 
comprises  the  bacteria  proper;  it  is  certainly  the  most  abundant 
of  the  three,  and  in  some  respects  it  is  the  most  important.  It 
is  with  the  bacteria  that  we  are  chiefly  concerned  in  this  work. 
Bacteria  have  sometimes  much  the  same  shape  as  yeasts.  The 
chief  distinction  between  them  is  their  method  of  multiplication. 
Instead  of  budding  they  multiply  by  fission.  The  bacterium 
elongates  a  little,  and  then  divides  into  two  equal  -<-\  ^ — n 

halves  at  once  (Fig.  5).     Hence  the  name  fission  — 

fungi.  Bacteria  are  also,  as  a  rule,  smaller  than  . — .  /"^^^ 
yeasts,  frequently  not  more  than  1/25000  of  an  ^^^  \.^ 
inch  in  diameter.     The  size  would  make  it  possible  {"^^ 

for  8,000,000,000  to  be  crowded  into  a  mass  no  ^-^V/ 

larger  than  a  pinhead;  and  we  can,  therefore,  easily  si^wing~mrthod 
understand  that  there  may  be  100,000,000  in  a  ofdivision  by 
drop  of  milk.  Occasionally,  however,  there  are 
larger  bacteria  and  smaller  yeast  cells.  While  the  size  is  no  sure 
criterion  between  the  two,  when  one  finds,  under  the  microscope, 
rather  large  round  or  oval  plants,  he  is  pretty  safe  in  calling  them 
yeasts,  while  the  smaller  ones  he  may  call  bacteria.  But  it  is 
necessary,  in  some  cases,  to  study  the  method  of  reproduction  before 
one  can  with  certainty  distinguish  yeasts  from  bacteria. 

This  group  of  bacteria  is  of  such  primary  importance  to  our 
study  that  we  must  learn  further  facts  concerning  their  classifica- 
tion and  characters. 


GENERAL  CHARACTERS  OF  BACTERIA. 

Colonies. — Bacteria  are  so  minute  that  they  cannot  be  handled 
as  individuals,  but  must  be  treated  in  masses.  One  of  the  primary 
difficulties  in  the  study  of  these  organisms  has  been  to  get  masses 
of  bacteria  that  would  be  large  enough  to  handle,  and  yet  would 
contain  only  one  kind  of  bacteria.  Such  masses  are  called  pure 
cultures,  and  it  was  this  difficulty  in  procuring  pure  cultures  that 


lO  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

for  a  long  time  prevented  the  development  of  the.  science.  Bacterio- 
logical study  to-day  commonly  begins  with  cultivating  the  bacteria, 
i.e.,  allowing  them  to  grow  in  some  medium  adapted  to  them  until 
they  become  abundant  enough  to  be  handled  in  bulk.  To  prevent 
the  mixing  of  the  different  kinds  that  may  be  in  the  material  we  are 
studying,  they  are  usually  grown  in  a  solid  or  jelly-like  medium, 
which  holds  the  individuals  fast  in  one  spot.  As  the  individuals 
multiply  in  this  solid  medium,  they  are  unable  to  separate  from  each 
other;  so  they  remain  in  little  clusters  which  in  time  become  large 

enough  to  be  seen  without  a  microscope. 
Such  clusters  are  called  colonies,  and 
figures  of  some  of  them  are  shown  in  Fig.  6. 
The  shape  and  appearance  of  the  colonies 
produced  by  different  kinds  of  bacteria  are 
often  very  different,  showing,  indeed,  greater 
Fig.  6.-Colonies  of  bacteria,  varieties  than  can  be  seen  in  the  bacteria 
themselves  with  a  microscope.  As  a  result  the  shape  and  appear- 
ance of  these  colonies  are  often  used  to  separate  the  numerous 
bacteria  from  each  other  and  to  classify  them.  A  colony,  when  it 
comes  from  the  multiplication  of  a  single  individual  bacterium,  is 
made  of  one  kind  of  bacteria  only.  This  colony  may  easily  be 
picked  out  with  a  sterile  needle,  and  when  properly  placed  in 
another  culture  medium  it  becomes  a  pure  culture.  The  starting- 
point  in  practical  bacteriological  study  is  thus  the  colony  rather 
than  the  individual  bacterium  (see  Laboratory  Work). 

Form  of  Bacteria. — Bacteria  are  of  three  quite  different  shapes, 
but  are  all  very  simple,  i.  Simple  spheres  (see  Fig.  7,  a).  Such 
spherical  forms  are  called  Cocci.  In  common  microscopical 
preparations  no  internal  structure  can  be  seen,  the  bacteria  appearing 
as  deeply  stained  balls.  The  Cocci,  however,  differ  somewhat 
in  their  method  of  growth,  thus  enabling  the  microscopist  to  distin- 
guish different  kinds,  as  will  be  mentioned  presently.  2.  The  rod- 
formed  bacteria  (Fig.  7,  h).  These  organisms  are  longer  than  they 
are  broad,  sometimes  only  slightly  so,  but  at  other  times  very  much 
longer,  forming,  indeed,  long,  slender  threads.  3.  The  spiral- 
formed   bacteria.     These    are   either   long,  coiled    spirals,  or  very 


GENERAL    CHARACTERS    OF   BACTERIA. 


II 


short  ones,  with  only  a  single  turn  (Fig.  7,  c).     This  type  is  of  less 
importance  than  the  others. 

Motility  of  Bacteria. — The  next  point  of  distinction  among 
bacteria  is  based  upon  their  motility.  Some  bacteria  are  capable 
of  an  active  swimming  motion,  others  are  stationary.  The  motion 
is  produced  by  minute,  extremely  delicate,  vibrating  hairs,  called 


Fig.  7. — General  shape  of  bacteria,     a,  spheres;  &,  rods;  c,  spirals. 

flagella  (Fig.  8).  The  flagella  are  so  delicate  that  they  cannot  often 
be  seen  in  the  living  bacteria,  and  they  do  not  stain  by  the  ordinary 
method  of  staining.  Therefore,  they  are  never  seen  in  the  usual 
microscopic  preparations.  They  may  be  seen  by  special  methods, 
but  these  are  so  difficult  that  the  beginner  cannot  use  them  satisfac- 
torily.    The  question  of  their  motility  is,  however,  usually  determ- 


FiG.  8.— Showing  bacteria  with  flagella;  a,  peritrichic;  6,  lophotrichic;  c,  monotrichic. 


ined  without  staining,  by  the  study  of  the  living  bacteria  (Experiment 
No.  8).  These  flagella  are  differently  distributed  upon  different 
bacteria.  Sometimes  there  is  a  single  one  on  the  end  of  a  rod 
(Fig.  8,  c) — monotrichic;  sometimes  a  small  tuft  at  one  or  both  ends 
of  a  rod  (Fig.  8,  b) — lophotrichic;  and  sometimes  there  is  a  cover- 
ing of  flagella  over  the  whole  body  of  the  bacterium  (Fig.  4,  a)  — 
peritrichic. 


12  THE    GENERAL    CHARACTERS    OF   MICROORGANISMS. 

Classification. — Based  upon  the  distinctions  thus  mentioned, 
the  bacteria  are  divided  into  groups: 

Spherical  Bacteria: 

Dividing  in  one  plain,  so  as  to  form  chains  (Fig.  9,  a),  Strepto- 
coccus. 

Dividing  in  two  plains,   and  not  forming  chains  (Fig.  9,  h), 
Micrococcus. 

Dividing  in  three  plains,  and  forming 
cubical  masses  (Fig.  9,  c),  Sarcina. 

Rod-shaped  Bacteria: 

With     flagella     and     consequently 
motile  (Fig.  8)*,  Bacillus. 
Without  flagella  and  consequently 
Fig.    9.— Showing    different  non-motile  (Fig.  7,  h) ,  Bacterium. 

types  of  COCCI,  a,  &,  and  c,  Micro-  .  \     o    /  ^     /  :> 

cocci;    d,  Streptococci;    e,  Sar-  With    a    single    flagellum,    Pseudo- 

cina.  _„  _  _,  ^  _ 

monas. 
Spiral  Bacteria  (Fig.  3,  a),  Spirillum. 

The  genus  Bacillus  is  further  divided  as  follows: 

Bacilli  with  only  one  flagellum  (Fig.  8,  c)   are  named  Mono- 

trichic  Bacilli,  or  Pseudomonas. 

Bacilli  with  one  flagellum  at  each  end,  Microsporon. 

Bacilli  with  a  tuft  of , flagella  at  one  end  (Fig.  8,  b),  are  called 

Lophotrichic  Bacilli. 

Bacilli  with  flagella  over  the  whole  body  (Fig.  8,   a)  are  called 

Peritrichic  Bacilli. 

Higher  Bacteria  (Cladothrix,  Leptothrix,  Streptothrix,  Actinomyces) 
(Fig.  10). 
Under  this  head  are  included  a  few  forms  of  fungi  which  re- 
semble other  bacteria  in  some  respects,  but  differ  in  others.     They 
are  composed  of   threads   which   are   commonly  larger  than   the 

*Unfortunately  bacteriologists  are  not  agreed  to-day  in  regard  to  the 
use  of  the  terms  above  given.  The  names  Bacillus  and  Bacterium  are  not 
always  used  as  here  stated,  and  recent  classification  of  the  spherical  forms 
recognizes,  in  addition  to  the  names  given, three  others,  Diplococcus,  Meta- 
coccus,  and  Ascococcus.  This  variation  in  nomenclature  results  in  great 
confusion.  In  the  absence  of  any  well-accepted  classification  to-day,  we 
shall  in  this  book  use  the  names 'as  above  defined. 


GENERAL    CHARACTERS    OF   BACTERIA. 


13 


threads  of  bacteria,  and  .which  may  show  frequent  branching,  a 
characteristic  not  usual  in  bacteria.  They  also  have  a  peculiar 
method  of  forming  reproducing  bodies.  The  group  is  not  one  of 
very  great  importance.  One  type  of  Streptothrix  is  extremely 
abundant  in  soil  and  appears  as  round,  white  opaque  colonies  with 
an  extensive  brown  halo  upon  the  plates  described  in  Experiment 
No.  24. 

Thus  it  will  be  seen  that  the  term  bacteria  applies  to  the  whole 
group  of  organisms  that  multiply  by  division,  the  study  of  which 
constitutes  the  study  of  bacteriology,  while 
the  term  Bacterium  refers  to  a  single  division 
of  the  group,  viz. :  the  non-motile,  rod  forms. 
The  term  Bacillus  should  apply  to  motile 
forms  only.  The  names  Bacillus  and  Bac- 
terium are  sometimes  confused;  for  example, 
the  tubercle  bacillus,  according  to  the  above 
classifiication,  is  a  Bacterium,  since  it  is  non- 
motile;  and  indeed  recent  study  indicates  that 
it  belongs  to  the  group  of  higher  fungi;  but  the 
name  bacillus  was  given  it  years  before  the 
above  distinctions  were  recognized,  and  we 
will  still  use  the  common  name.  Some  other 
bacteria,  named  twenty  years  ago,  retain  their 
earlier  names  in  some  books,  but  they  are 
slowly  having  their  names  brought  into 
harmony  with  the  above  distinctions. 

The  term  Coccus  is  applied  to  any  spherical  organism  of  the 
group  bacteria. 

This  classification  gives  only  what  are  recognized  as  the  genera 
of  bacteria.  A  further  classification  of  the  group  into  species  is  at 
the  present  time  in  a  condition  of  the  greatest  confusion.  Many 
hundred  varieties  have  been  described  by  different  bacteriologists, 
but  there  is  great  difficulty  in  giving  any  distinctive  description  of 
such  minute  organisms,  which  have  so  few  characters;  and  it  is 
quite  uncertain  whether  these  many  hundred  described  species 
represent  distinct  forms  or  whether  they  should  be  reduced  to  a 


Fig.  10, — Actinomy- 
ces, a,  a  small  colony; 
h,  bingle  rods  {Bostrom). 


14  THE    GENERAL   CHARACTERS    OF   MICROORGANISMS. 

much  smaller  number  of  species.  It  is  frequently  uncertain  whether 
a  species  described  by  one  bacteriologist  is  the  same  as  that  de- 
scribed by  another  under  the  same  name.  The  difficulties  in  the 
way  of  a  proper  description  and  classification  of  the  species  of 
bacteria  have  hitherto  been  insurmountable,  and  at  the  present 
time  the  subject  is  in  such  extreme  confusion  that  no  one  except  an 
expert  can  understand  it.  Fortunately  this  confusion  of  species 
is  of  no  importance  for  our  purpose.  Agricultural  bacteriology  is 
not  at  present  concerned  with  the  problem  of  the  species.  All  that 
it  is  necessary  for  us  to  know  in  connection  with  our  subject  will  be 
referred  to  in  the  separate  sections  in  the  following  pages,  and  the 
subject  of  the  classification  of  bacteria  may  be  left  without  further 
consideration. 

Multiplication  of  Bacteria. — As  already  mentioned,  the 
primary  method  of  the  multiplication  of  bacteria  is  by  simple 
division.  Bacteria  are  so  minute  that  it  seems  strange  to  assign 
to  them  much  of  a  part  to  play  in  nature's  processes.  But  their 
extraordinary  power  of  multiplication  gives  them  unlimited  possi- 
bilities. 

The  elongation  of  a  rod  and  its  division  into  two  parts,  followed 
by  a  repetition  of  the  process,  may  be  extremely  rapid.  Frequently 
it  does  not  take  more  than  half  an  hour  for  the  whole  phenomenon 
to  take  place,  and  sometimes  even  less  time  is  required.  Such 
division,  in  geometrical  ratio,  results  in  an  increase  in  numbers 
that  is  almost  inconceivably  great.  If  a  division  once  an  hour 
could  be  maintained  for  twenty-four  hours,  there  would  be  pro- 
duced, as  the  offspring  of  a  single  bacterium,  some  seventeen  million 
descendants,  and  in  five  days  there  would  be  a  mass  sufficient  to  fill 
the  oceans.  This  rate  is,  manifestly,  iiot  continued  for  any  great 
length  of  time,  or  the  world  would  be  full  of  them;  their  growth  is 
checked  by  lack  of  food,  and  still  more  by  the  substances  they  se- 
crete, which  act  as  poisons.  But  this  possibility  of  reproduction 
represents  an  almost  unlimited  power,  constantly  curbed  by  the 
lack  of  proper  conditions.  Bacteria  may  thus  be  looked  upon  as 
possessing  a  wonderful  possibility  of  reproduction,  a  force  of  in- 
conceivable magnitude,  held  more  or  less  in  check  by  adverse  condi- 


I 


GENERAL    CHARACTERS    OF   BACTERIA.  1 5 

tions,  but  ever  ready  to  exert  their  influence  when  the  conditions  are 
favorable.  Since  they  are  feeding  during  their  growth,  they  must 
produce  profound  changes  in  the  material  upon  which  they  feed. 
It  is  this  reserve  force,  possessed  in  greater  or  less  degree  by  all 
bacteria,  which  makes  them  such  wonderful  and  powerful  agents 
in  producing  the  great  changes  in  nature  which  we  are  now  forced  to 
attribute  to  them. 

Production  of  Spores. — There  is  another  method  of  produc- 
ing new  individuals,  an  understanding  of  which  is  necessary  to  a 
knowledge  of  bacteria.  This  is  the  production  of  spores,  and  is 
illustrated  in  Fig.  ii,  a-e.  The  bacterium  there  figured  consists  of 
a  rod.  The  contents  of  one  of  the  rods  ^^  ^^  ^...^ 
collects  itself  in  a  spherical  or  oval  body  in  <^  3  c  cT  V^ 
the  center.     This  later  breaks  out  of  the  V.^        ^ 

rod,  the  rest  of  the  individual  then  dying      4^^J>o  ¥ 

and  disappearing.     The  oval  body  itself  is         ^T^  y^^^  /in  ^ 
a  spore,  and  is  capable,  when  placed  under  ^  ff   V^ 

proper  conditions,  of  developing  into  a  new  "^    ^  ^^ 

rod,  e.     Inasmuch  as  only  a  sinde  spore 

-  .11  .  .     .  Fig.  II. — Spore  formation. 

arises  from  a  smgle  bacterium,  it  is  not  a  a  to  «,  stages  in  spore  for- 
multiplication.  Its  purpose  is  not  so  much  °^^t^°^  ^^^  germination. 
to  increase  the  number  of  individuals  as  to  enable  the  bacteria  to 
endure  adverse  conditions  without  being  killed.  The  ordinary  bac- 
teria are  likely  to  be  killed  by  being  dried,  and  will  readily  succumb 
to  moderate  heat,  a  temperature  of  165°  F.*  being  sufficient  to  kill 
almost  any  of  them.  But  these  spores  are  covered  with  a  hard  case 
which  enables  them  to  resist  the  conditions  which  the  active,  growing, 
and  multiplying  forms  cannot  resist.  They  may  be  completely  dried 
for  months,  and  even  years,  and  still  retain  their  vitality.  They  may 
be  heated  very  much  hotter  than  the  active  forms  without  injury;  in- 
deed, some  of  these  spores  may  be  in  boiling  water  for  many  minutes 
— an  hour  or  longer — without  having  their  vitality  destroyed,  since, 
if  the  spores  are  subsequently  cooled,  they  are  capable  of  germinating 
and  growing  into  new  bacteria.  As  a  result  of  this  it  will  follow 
that,  while  it  is  very  easy  to  kill  ordinary  bacteria  by  heat,  it  is 
*  Temperatures  used  in  this  book  always  refer  to  the  Fahrenheit  scale. 


l6  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

far  more  difficult  to  destroy  spores.  Many  species  of  bacteria 
produce  such  spores  (Fig.  ii,  /) ;  others  do  not,  and  hence  some  are 
much  more  easily  killed  by  heat  than  others.  Milk,  for  example, 
contains  many  kinds  of  bacteria.  By  the  simple  boiling  or,  indeed, 
the  heating  of  the  milk  to  a  temperature  of  i6o°  F.,  a  vast  majority 
of  the  bacteria  are  killed;  but  the  few  spores  that  may  chance  to 
be  in  the  milk  are  not  thus  killed,  and  subsequently  these  will  be 
able  to  develop.  If  milk  contains  spore-bearing  bacteria,  it  cannot 
be  sterilized  by  boiling;  and,  since  it  almost  always  does  contain 
them,  boiling  is  not  sufficient  to  sterilize  it.  This  phenomenon  of 
the  high  resisting  powers  of  spores  must  always  be  borne  in  mind 
in  all  problems  of  sterilizing. 

Relations  to  Conditions. — Temperature. — The  rate  of  multipli- 
cation of  bacteria,  yeasts,  and  molds  depends  upon  the  temperature. 
At  freezing  they  do  not  grow  at  all.  As  the  temperature  rises  above 
freezing  they  begin  to  multiply,  and  their  rate  of  multiplication 
increases  as  the  temperature  rises,  up  to  a  certain  point  which  is 
the  optimum  temperature.  If  the  temperature  rises  still  higher, 
the  rate  declines  and  finally  growth  stops.  If  heated  still  more, 
the  organisms  are  killed.  The  lowest  temperature  at  which  they 
will  grow,  the  minimum  temperature,  varies  with  different  species. 
Some  will  grow  at  a  temperature  only  just  above  freezing,  at  ^2>°  F., 
while,  at  the  other  extreme,  some  will  not  grow  at  temperatures 
lower  than  120°  to  140°  F.  The  optimum  temperature  also  varies. 
Some  species  grow  best  at  moderately  low  temperatures,  60°  or 
as  low  as  50°  F.,  while  others  flourish  best  at  a  temperature  from 
90°  to  100°  F.  When  the  temperature -is  above  100°,  most  bacteria 
grow  less  rapidly  than  when  it  is  a  little  lower,  while  at  a  slightly 
higher  temperature  they  cease  growing.  A  few  species,  however, 
grow  best  at  unexpectedly  high  temperatures,  some  having  been 
found  flourishing  at  140°  or  even  higher.  These  peculiar  bacteria 
are  called  thermophiles.  How  they  can  find  conditions  in  nature 
warm  enough  for  their  growth  is  a  question. 

The  death  temperature  is  a  factor  of  great  importance,  since  it  is 
so  closely  associated  with  the  matter  of  sterilization  by  heat.  Most 
bacteria,  when  in  an  active  condition,  are  killed  by  a  temperature 


GENERAL    CHARACTERS    OF    BACTERIA. 


17 


'^'^^Sw^issa*^*!! 


^^K>f  140°  if  maintained  for  half  an  hour.  At  this  temperature, 
however,  they  die  slowly;  a  temperature  of  150°  destroys  them  more 
l^jrapidly  still,  while  a  temperature  from  170°  to  180°  is  proportionately 
^^Bwiore  effective.  A  total  destruction  of  bacteria,  including  their 
spores,  can  be  brought  about  only  by  a  temperature  above  that 
of  boiling  water,  and  this  is  usually  accomplished,  in  the  case  of 
liquids,  in  a  closed  chamber  where  the 
team  can  be  generated  under  con- 
siderable pressure.  If  the  steam  is 
Illowxd  to  collect  in  such  a  chamber 
t  a  pressure  of  15  pounds,  the  tem- 
erature,  then,  will  be  about  240°. 
^his  temperature,  kept  up  for  half  or 
three-quarters  of  an  hour,  destroys 
even  the  most  resisting  spores.  Lab- 
oratories usually  have  a  small  appa- 
ratus designed  for  this  purpose,  called 
an  autoclav  (Fig.  12),  and  this  is  used 
constantly  for  sterilizing  liquids. 

Sterilization. — This  is  a  process 
closely  related  to  the  question  of  death 
temperatures.  Sterilization  is  some-, 
times,  to  be  sure,  brought  about  by 
adding  poisonous  chemicals  to  the 
material  to  be  sterilized;  but  more 
commonly,   and    almost    universally, 

when  we  are  dealing  with  food  products,  sterilization  is  accomplished 
by  heat.  If  a  material  to  be  sterilized  contains  only  active  organisms, 
it  might  be  accomplished  by  subjecting  it  to  a  moderately  low 
heat,  140°  to  150°  F.  But  it  is  almost  always  a  fact  that  anything 
which  we  wish  to  sterilize  is  likely  to  contain  spores,  and,  since 
these  withstand  a  higher  temperature,  no  moderate  heat  will 
accomplish  the  purpose.  Even  boiling  is  not  sufficient  to  destroy 
spores,  so,  to  be  sure  of  complete  sterilization,  a  temperature  above 
boiling  is  necessary.  If  the  object  is  a  solid  that  can  bear  heat 
it  is  simply  heated  at  about  300°  F.  for  an  hour  or  so.     If  it  is  a 


Fig,  12, — An  autoclav  used  in 
sterilization  of  liquids  under  pressure 
{Eyr). 


l8  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

liquid  it  is  placed  in  an  autoclave  (Fig.  12),  and  heat  is  applied  until 
there  is  a  steam  pressure  from  10  to  15  pounds.  This  produces  a 
temperature  sufl&cient  to  destroy  spores. 

Sometimes  it  is  desirable  to  sterilize  liquids  that  will  not  stand 
these  high  temperatures,  as,  for  instance,  gelatin  (Experiment  No. 
1 2) .  This  can  be  accomplished  by  discontinuous  heat.  The  material 
is  heated  to  about  180°  F.,  or  more  commonly  to  boiling.  It  is  then 
cooled  and  allowed  to  stand  twenty-four  hours  in  a  warm  place. 
The  heat  has  destroyed  the  active  bacteria,  but  has  not  killed  the 
spores,  which,  during  the  twenty-four  hours,  will  germinate  and 
grow  into  active  bacteria.  Heat  is  again  applied  as  before,  and  this 
time  any  active  bacteria  that  may  have  come  from  the  germination 
of  the  spores  are  killed.  The  material  is  allowed  to  stand  another 
day  so  that  any  spores  that  may  have  failed  to  germinate  the  first 
day  may  grow,  then  heat  is  applied  again.  Experience  shows  that 
three  heatings  of  this  sort  will  destroy  all  the  organisms  and 
sterilize  the  liquid.  To  be  successful  in  this  method  it  is  necessary 
that  the  interval  between  the  heatings  should  be  long  enough  for 
the  spores  to  germinate,  but  not  long  enough  for  the  bacteria 
arising  from  them  to  form  any  more  spores.  Twenty-four  hour 
intervals  have  been  found  to  be  the  best. 

Relation  to  Cold. — While  heat  will  destroy  all  bacteria,  cold  will 
not  do  so.  It  is  practically  impossible  to  destroy  the  life  of  bacteria 
by  freezing,  at  least  with  any  certainty;  for  no  matter  how  low  the 
degree  of  temperature  used,  the  life  of  some  of  these  organisms  seems 
to  be  totally  resistant.  Experiments  have  shown  that  bacteria 
cooled  to  the  temperature  of  liquid  air,  or  even  liquid  hydrogen,  are 
not  all  killed,  but  after  being  warmed  are  still  able  to  germinate. 
Although  these  extremes  of  temperature,  do  not  destroy  all  bacteria, 
the  simple  matter  of  freezing  and  thawing  will  kill  a  great  number 
of  them.  If  water  containing  a  large  number  of  bacteria  is  frozen 
and  subsequently  thawed,  the  bacteria  will  be  found  much  reduced 
in  numbers,  although  they  are  not  by  any  means  all  killed.  When, 
therefore,  water  is  contaminated  by  sewage  containing  typhoid 
bacteria,  and  ice  is  collected  from  it  for  domestic  purposes,  the 
typhoid  bacteria  may  still  be  found  alive  in  the  ice.     Such  ice  may 


GENERAL    CHARACTERS    OF    BACTERIA. 


19 


still  be  a  source  of  danger.  But  it  must  also  be  remembered  that 
freezing  destroys  a  very  large  proportion  of  these  germs,  so  that  the 
danger  from  the  ice  is  far  less  than  from  the  water  before  it  was 
frozen. 

Relation  to  Air. — Nearly  all  living  organisms  require  air,  and  it 
was  formerly  supposed  that  nothing  could  live  without  it.  Certain 
types  of  bacteria,  however,  are  able  to  live  without  air.  Indeed, 
some  species,  while  they  grow  readily  if  they  have  no  contact  with 
air,  fail  to  grow  at  all  when  the  slightest  amount  of  air  is  present, 
growing  only  in  the  absence  of  oxygen.  This  type  of  bacteria  is 
spoken  of  as  anaerobic.  At  the  other  extreme,  there  is  a  long  list  of 
bacteria  which  can  grow  only  in  the  presence  of  air',  failing  to  grow 
if  they  do  not  have  oxygen  at  their  command.  This  is  the  type  of 
aerobic  bacteria.  Between  the  two  is  an  intermediate  group 
capable  of  growing  either  in  the  air  or  out  of  contact  with  it,  and 
these  are  spoken  of  as  facultative  anaerobic. 

Relation  to  Moisture. — Bacteria  will  grow  only  in  the  presence  of 
considerable  quantities  of  moisture;  indeed,  they  demand  more 
moisture  than  most  organisms.  Some  of  them  will  hardly  grow  at 
all  unless  there  is  30  per  cent,  of  moisture  in  the  material  in  which 
they  are  living,  and  even  then  the  growth  is  slow.  On  the  other 
hand,  they  flourish  most  luxuriantly  in  localities  where  the  water  is 
from  90  to  100  per  cent.  Hence,  as  materials  dry,  bacteria  will 
cease  to  grow  in  them,  and  any  substance  that  can  be  dried  can  be 
thoroughly  protected  from  their  action.  This  explains  why  dried 
fish  and  dried  meat,  fruits,  dried  milk,  etc.,  will  keep  indefinitely. 
The  drying,  however,  does  not  actually  kill  the  bacteria,  for  al- 
though they  do  not  grow  when  the  water  is  extracted  from  them, 
they  may  remain  alive  for  weeks,  months,  or  even  years.  In  other 
words,  it  is  impossible  to  depend  upon  drying  as  a  means  of  de- 
stroying bacteria,  for,  while  many  individuals  will  fail  to  live,  many 
others  do  not  seem  to  be  injured  at  all  by  the  drying,  and  are  capable 
of  resuming  life  again  as  soon  as  they  find  moisture. 

Yeasts  are  much  like  bacteria  in  respect  to  need  for  water,  and 
will  not  grow  unless  the  water  content  is  high.  But  other  fungi 
with  which  we  are  concerned,  the  molds  and  mushrooms,  can  get 


20  THE    GENERAL    CHARACTERS    OF    MICROORGANISMS. 

along  upon  a  smaller  amount  of  water.  Substances  that  are  too 
dry  to  putrefy  may  be  spoiled  by  molding;  hence  it  is  much  more 
difficult  to  preserve  certain  kinds  of  partly  dry  food  from  moulding 
than  from  decaying.  Flour,  in  a  flour  barrel,  may  become  musty 
from  the  development  of  molds,  but  it  will  hardly  show  signs  of 
decay  or  putrefaction  unless  it  becomes  actually  wet. 

Relation  to  Food. — Bacteria  and  the  other  fungi  feed  upon  an 
immense  variety  of  foods.  A  few  of  them  are  able  to  nourish 
themselves  upon  mineral  matter,  and  some  can  gain  their  necessary 
carbon  from  the  small  amounts  of  carbon  dioxide  in  the  air,  re- 
sembling, in  this  respect,  the  green  plants  that  are  engaged  in  build- 
ing up  starch  and  other  organic  compounds.  This  class  of  bacteria 
is  of  great  theoretical  interest  and  doubtless  of  much  importance  in 
nature.  But  the  vast  majority  of  microorganisms  are  unable  to  use 
mineral  foods,  requiring,  like  animals,  to  be  fed  with  organic  food. 
In  other  words,  they  require  for  their  life  the  same  kind  of  foods 
that  the  animal  kingdom  requires.  They  can  consume  proteids, 
starches,  sugars,  fats,  woody  tissue,  and,  in  short,  almost  anything 
that  is  found  in  the  bodies  of  animals  and  plants.  The  different 
varieties  of  microorganisms  do  not  all  flourish  upon  the  same  kind  of 
food.  Some  seem  to  be  able  to  live  upon  a  large  variety  of  sub- 
stances, while  others  demand  particular  foods.  While  almost  any 
kind  of  proteids  will  serve  for  the  sustenance  of  the  common  putre- 
factive bacteria,  the  tubercle  bacillus  does  not  flourish  well  anywhere 
outside  the  living  body,  and,  if  it  is  to  be  cultivated  in  the  laboratory, 
it  demands  a  very  special  kind  of  culture  medium.  But  speaking 
in  broad  terms,  the  three  classes  of  organisms  with  which  we  are 
concerned  in  our  subject,  seem  to  be  particularly  adapted  to  different 
kinds  of  food.  Bacteria  have  special  relations  to  proteid  foods, 
like  lean  meat,  egg  albumen,  gluten  of  wheat,  etc.,  and  if  substances 
of  this  nature  are  consumed  by  microorganisms,  it  is  commonly  by 
bacteria.  Yeasts,  on  the  other  hand,  have  a  special  fondness  for 
sugars  and,  therefore,  for  starches,  which  are  easily  changed  into 
sugars.  The  larger  fungi  may  feed  upon  either  proteids  or  sugars, 
but  they  have  special  relations  to  the  woody  tissues  and  celluloses  of 
vegetable  structures.     This  classification  of  the  foods  upon  which 


GENERAL    CHARACTERS    OF    BACTERIA.  21 

these  different  forms  subsist  is  by  no  means  exact,  for  each  group 
may  contain  members  that  make  use  of  all  kinds  of  foods;  but, 
generally  speaking,  the  higher  molds  and  mushrooms  attack  the 
,  harder  plant  tissues,  the  yeasts  attack  sugars,  while  the  bacteria  are 
especially  concerned  in  the  destruction  of  proteids. 

The  food  which  bacteria  consume  may  be  either  living  or  dead 
when  they  attack  it.  In  the  case  of  most  bacteria  the  organisms 
are  unable  to  feed  upon  the  material  while  it  is  alive.  If  bacteria, 
for  example,  are  placed  upon  living  muscle,  they  are  usually  unable 
to  attack  it  and  soon  die;  but  if  they  are  placed  upon  the  same 
muscle  after  it  is  dead,  they  feed  upon  it  readily  and  cause  it  to 
putrefy.  Those  organisms  which  feed  upon  lifeless  bodies  include 
the  vast  majority  of  bacteria.  There  are,  however,  other  species 
that  are  capable  of  living  upon  the  bodies  of  animals  and  plants 
while  these  are  still  alive.  Inasmuch  as  they  can  feed  upon  living 
organisms  they  are  liable  to  produce  disease  and  constitute  in  general 
the  disease  bacteria.  Bacteria  feeding  upon  living  animals  and 
plants  are  called  parasites.  Bacteria  feeding  upon  dead  animals 
and  plants  are  called  saprophytes.  Both  saprophytes  and  parasites 
are  of  great  importance. 


CHAPTER  II. 

FERMENTATION,  PUTREFACTION,  AND  DECAY. 

THE  NATURE  OF  THE  ACTIVITIES  OF  MICRO- 
ORGANISMS. 

Everyone  at  all  familiar  with  nature  must  realize  that  there  is 
constantly  going  on,  in  earth,  water,  and  air,  an  uninterrupted  series 
of  slow  changes.  Rocks  disintegrate;  fruits  decay  and  their  juices 
ferment;  vegetables  rot;  animal  bodies  putrefy;  milk  sours;  cheese 
ripens;  the  soil  becomes  contaminated  by  the  decaying  waste  of 
sewage  and  then  purifies  itself;  streams  become  foul  and  grow  clear 
again;  even  tree  trunks  rot  and  disappear.  These  and  hosts  of 
other  kindred  phenomena  are  matters  of  such  every-day  occurrence 
that  we  scarcely  ever  stop  to  think  what  they  mean  or  how  they  are 
brought  about.  But  it  is  with  these  phenomena  that  we  are  chiefly 
concerned  in  the  study  of  germ  life  on  the  farm.  These  changes 
have  one  characteristic  in  common:  they  are  all  the  result  of  chemical 
decomposition.  Until  recently  it  has  been  supposed  that  they  are 
the  result  of  purely  chemical  forces.  The  chemical  agency  of 
oxidation,  especially  the  so-called  slow  oxidation,  has  been  supposed 
to  account  for  most  of  them. 

But  it  has  been  proved  by  modern  study  that  pure  chemical 
forces  are  not  able  to  produce  these  phenomena,  and  that  many  a 
process  formerly  called  slow  oxidation  is  not  the  result  of  chemical, 
but  rather  of  biological  forces.  If  microorganisms  can  be  kept  from 
them,  fruits  will  not  decay,  vegetables  will  not  rot,  and  many  other 
changes  will  fail  to  appear.  Most  of  the  slow  changes  referred  to 
are  the  result  of  the  action  of  the  great  class  of  fungi,  foremost 
among  which  stand  the  bacteria  and  yeasts.  The  reason  why  these 
organisms  are  so  closely  associated  with  phenomena  is  because  they 
are  capable  of  bringing  about  profound  chemical  changes. 

22 


CHEMICAL    CHANGES    PRODUCED    BY   MICROORGANISMS.  23 

From  facts  already  considered  it  will  be  evident  that  micro- 
organisms have  properties  that  certainly  fit  them  for  this  work. 
They  feed,  not  upon  minerals,  as  a  rule,  but  upon  the  organic 
material  in  nature;  and  each  kind  of  organic  food,  proteid,  sugar, 
starch,  wood,  etc.,  is  especially  subject  to  the  attack  of  one  of  the 
classes  of  fungi.  We  have  seen  also  what  inconceivable  powers  of 
multiplication  are  possessed  by^bacteria,  and,  while  the  other  or- 
ganisms do  not  grow  so  fast,  they  are  all  rapid  growers.  While  they 
are  growing  and  multiplying  with  such  vigor,  they  are  producing 
profound  changes  in  the  chemical  nature  of  the  food  upon  which 
they  are  feeding. 

CHEMICAL   CHANGES  PRODUCED   BY   MICRO- 
ORGANISMS. 

The  chemical  changes  thus  brought  about  are  very  numerous. 
The  chemist  of  to-day  has  hardly  begun  to  study  them,  and  his 
knowledge  is,  as  yet,  very  fragmentary.  Only  a  very  few  of  them 
are  understood,  and  in  regard  to  the  simplest  of  these  our  knowledge 
of  the  phenomena  is  yet  lacking  in  many  important  respects.  A  few 
only,  bearing  directly  upon  the  subject  of  agriculture,  will  be  ex- 
plained.    They  may  be  grouped  under  two  quite  distinct  heads. 

Synthetic  Processes.  Anabolism. — These  consist  in  the 
building  of  complex  bodies  out  of  simpler  ones.  The  fundamental 
importance  of  synthetical  processes  to  the  continuance  of  life  is 
evident  enough.  The  animal  kingdom,  in  general,  demands  complex 
compounds  as  foods,  and  cannot  live  upon  the  simple  compounds 
found  in  the  air  and  the  soil,  like  carbonic  dioxide,  nitrogen,  am- 
monia, etc.  (CO2,  N,  NH3).  In  order  that  animals  may  use  the 
elements  existing  in  nature,  some  process  must  build  them  into 
complex  bodies.  This  is  largely  accomplished  by  the  green  plants 
that  furnish  animals  with  food.  But  even  these  plants  demand 
some  of  their  food  in  a  complex  form,  not  being  able,  for  example,  to 
use  nature's  free  nitrogen  store  in  the  atmosphere  until  it  has  been 
built  up  into  some  compound  like  nitric  acid.  The  constructive 
or  synthetic  processes  are  thus  of  fundamental  importance  to  the  life 


24        THE    NATURE    OF   THE    ACTIVITIES    OF    MICROORGANISMS. 

processes  of  both  animals  and  plants.  Among  the  chemical  changes 
which  are  brought  about  by  bacteria,  some  are  of  this  synthetic 
character. 

Analytical  Processes.  Decomposition:  Katabolism. — The 
most  noticeable  action  of  bacteria  is  that  of  decomposition.  The 
great  majority  of  them,  just  like  animals,  live  upon  complex  chemical 
foods,  and  these  compounds  are  broken  to  pieces  by  their  action  and 
reduced  to  simpler  molecules.  Acting  in  this  way,  the  fungi  are 
the  most  important  agents  in  nature  for  reducing  to  a  simpler  condi- 
tion the  great  quantity  of  organic  matter  which  would  otherwise 
accumulate  upon  and  within  the  soil,  or  in  bodies  of  water.  The 
chemistry  of  the  decomposition  of  organic  substances  is  still  in  its 
infancy,  and  as  yet  only  the  general  nature  of  the  changes  is  under- 
stood. The  decomposition  of  these  compounds  in  general  brings 
the  elements  back  to  simpler  conditions  and  nearer  to  the  form  in 
which  they  can  serve  as  food  for  ordinary  plants. 

Both  synthetical  and  analytical  processes  are  carried  on,  to  a 
certain  extent,  by  all  bacteria.  If  they  grow  and  multiply  they 
must  be  manufacturing  proteid  and  protoplasm  out  of  the  food  prod- 
ucts, for  each  new  bacterium  is  made  of  protoplasm.  This  build- 
ing of  protoplasm  is  a  synthetic  process,  and  is,  of  course,  character- 
istic of  all  growing  bacteria.  On  the  other  hand,  all  bacteria  like- 
wise produce  a  certain  amount  of  decomposition  of  the  materials 
which  serve  them  as  food,  giving  rise  to  simpler  products  as  excre- 
tions. But  while  all  bacteria  thus  perform  both  types  of  chemical 
change,  the  decomposition  activity  is,  in  general,  much  greater  than 
that  of  synthesis;  they  are  destructive  rather  than  constructive  agents. 

In  still  another  respect  the  chemical  changes  produced  by  bacteria 
are  two-fold.  In  some  cases  the  new  products  which  arise  are  of 
the  nature  of  excretions.  By  this  is  meant  that  certain  substances 
are  taken  into  the  bacteria  and  then  subjected  to  a  series  of  changes 
within  their  bodies.  These  changes  are  classed  together  under  the 
name  of  metabolism.  As  a  result  of  the  metabolic  changes  there 
arise  new  chemical  products  which  may  be  eventually  eliminated 
from  the  body  of  the  bacteria  as  excretions.  Some  of  the  new 
products  arising  in  a  mass  of  organic  material  undergoing  decompo- 


TYPES  OF  FERMENTATION  AND  DECAY.  25 

sition  by  bacteria  are  ■  thus  of  the  nature  of  excretions  {e.g., 
ptomaines) .  In  other  cases  the  new  chemical  bodies  are  apparently 
produced  entirely  outside  the  body  of  the  bacteria,  and  are  not 
in  any  sense  excreted  products.  In  such  cases  the  microorganisms 
excrete  substances  which  act  upon  the  outside  substances,  producing 
chemical  changes  in  them.  The  substances  thus  produced  are  not 
excretions,  but  by-products. 

TYPES  OF  FERMENTATION  AND  DECAY. 

The  various  processes  known  as  fermentation,  decay,  putre- 
faction, etc.,  are  all  closely  related,  and,  while  an  attempt  has  been 
made  to  distinguish  between  them,  no  real  logical  distinctions  can 
be  made.  They  are  all  progressive  chemical  changes  taking  place 
under  the  influence  of  organic  substances  which  are  present  in  small 
quantity  in  the  fermenting  mass.  They  are  fundamentally  of  the 
nature  of  chemical  decompositions  by  means  of  which  organic 
substances  are  broken  down  and  new  substances  are  formed. 
They  may  best  be  understood  by  the  consideration  of  three  examples 
illustrating  three  different  types: 

Alcoholic  Fermentation. — When  yeast  is  added  to  a  sugar 
solution,  it  grows  rapidly  and  soon  changes  the  sugar  into  carbon 
dioxide  and  alcohol.  The  sugar  is  probably  taken  into  the  body  of 
the  yeast  and  decomposed  into  these  two  products,  which  are  then 
liberated  from  the  yeast,  the  gas  appearing  as  bubbles  and  the  alcohol 
remaining  in  the  solution.  The  change  is  sometimes  expressed  by 
the  chemical  equation,  C6Hj2  06  =  2C2H60-|-2C02.  But  this 
equation  represents  only  the  end-products  and  by  no  means  correctly 
expresses  the  changes  that  occur.  This  fermentation  occurs  in 
malt  to  make  beer,  in  apple  juice  to  make  cider,  and  is  the  basis 
of  alcoholic  industries  in  general.  It  also  occurs  in  the  raising  of 
bread. 

The  Amyolytic  Fermentation. — If  a   little   saliva   is   mixed 

with  starch  and  water,  there  begins  at  once  a  conversion  of  the 

starch  into  sugar,  and  it  may  continue  until  all  the  starch  is  thus 

changed.     This    fermentation    is    also    expressed    by   a    chemical 

3 


26        THE   NATURE    OF   THE   ACTIVITIES   OF  MICROORGANISMS. 

equation,  C6Hjo05+H2  0  =  C6Hj2  06,  though  the  equation  certainly 
does  not  represent  the  change  that  goes  on.  Starch  is  known  not 
to  be  such  a  simple  molecule  as  CaHjoO^,  but  some  multiple  of  that 
formula,  and  probably  a  very  high  one.  Its  decomposition  into 
sugar  is  really  a  long  series  of  steps,  only  the  final  result  being 
partly  represented  in  the  equation.  This  fermentation  occurs  in 
food  after  it  is  mixed  with  saliva  in  the  mouth. 

Putrefaction  and  Decay. — If  any  proteid  body — meat,  eggs, 
or  the  like — be  left  for  some  time  exposed  to  the  air,  it  will  give  off 
unpleasant  odors,  for  it  is  undergoing  putrefaction  and  decay. 
These  two  processes,  though  frequently  considered  the  same,  are 
slightly  different.  Both  are  the  result  of  the  chemical  decomposition 
of  organic  compounds,  and  the  terms  are  commonly  applied  only 
to  the  decomposition  of  material  that  contains  proteids.  Both 
result  in  chemical  decompositions  which  are  very  complete,  and 
more  complex  and  indefinite  than  the  other  two  types  of  fermenta- 
tions. They  are  produced  by  microorganisms,  chiefly  bacteria, 
which  feed  upon  the  putrefying  mass,  taking  certain  atoms  out  of 
the  organic  molecules.  These  molecules,  thus  losing  some  of  their 
atoms,  change  their  chemical  nature.  The  remaining  atoms 
necessarily  rearrange  themselves  to  form  new  compounds  which  are 
simpler  in  structure.  The  distinction  between  putrefaction  and 
decay  consists  in  the  fact  that  decay  is  the  term  applied  to  decompo- 
sition in  the  presence  of  oxygen,  while  putrefaction  takes  place 
in  the  absence  of  oxygen.  The  former  is  much  more  complete 
than  the  latter,  resulting  in  the  more  complete  destruction  of  the 
substance  decomposed. 

Organized  and  Unorganized  Ferments.— These  three  ex- 
amples of  fermentation  are  very  different  from  one  another.  One 
seems  to  break  the  sugar  molecule  into  two  simple  portions,  carbonic 
acid  and  alcohol;  the  second  simply  adds  a  molecule  of  water  to  one 
of  starch;  while  the  third  results  in  a  complete  decomposition  of  a 
highly  complex  proteid  into  a  large  number  of  by-products,  both 
known  and  unknown.  But  in  some  important  respects  all  three 
agree.  Each  differs  from  ordinary  chemical  processes  in  several 
respects  and  all  agree  in  the  following  points: 


TYPES  OF  FERMENTATION  AND  DECAY.  27 

1.  They  are  all  cl6sely  associated  with  life  processes;  i.e.,  are 
brought  about  directly  or  indirectly  by  living  agents. 

2.  They  are  all  closely  dependent  upon  temperature,  ceasing 
at  low  temperatures  and  also  at  high  temperatures,  and  occurring 
with  vigor  within  limits  of  temperature  not  far  apart.  Most  of 
them  occur  most  vigorously  at  temperatures  between  80°  and  100°  F. 

3.  They  are  all  produced  by  the  stimulating  action  of  some 
special  body,  present  in  the  fermenting  material  in  a  quantity 
which  is  very  small,  considering  the  great  changes  produced. 

4.  These  bodies  (ferments)  are  all  rendered  inert  or  destroyed 
by  heat;  a  boiling  temperature  commonly  destroys  them  so  completely 
that  they  are  unable  to  renew  their  action  even  after  cooling.  Low 
temperatures  simply  check  their  activity,  which  they  are  able  to 
renew  if  warmed  again. 

5.  Their  action  is  completely  stopped  by  an  accumulation  of 
the  products  of  their  own  activity. 

If  we  ask  what  is  the  body  producing  the  action  (the  ferment), 
we  find  that  the  first  and  last  of  the  types  described  dififer  from  the 
second  in  one  radical  point.  Whereas  the  alcoholic  fermentation 
and  putrefaction  are  directly  produced  by  living  germs,  either  yeasts 
or  bacteria,  the  amyolytic  fermentation  is  not  produced  by  a  living 
organism,  but  by  some  non-living  substance  secreted  from  a  living 
being.  To  explain  this  a  brief  account  is  required  of  the  develop- 
ment of  our  knowledge  of  fermentations  in  general. 

Fermentations  have  been  known  for  centuries.  Even  in  ancient 
Egypt  the  production  of  alcohol  was  familiar.  Every  savage  tribe  has 
its  own  method  of  obtaining  alcohol  by  the  fermenting  of  fruit  juices, 
and  the  process  is  one  of  the  most  widely-known  changes  in  nature. 
For  a  time  it  was  regarded  as  a  putrefying  process,  the  yeasts  found 
in  the  fermented  material  being  looked  upon  as  an  impurity  which 
was  separated  from  the  rest.  The  chemical  nature  of  alcoholic  fer- 
mentation was  determined  early  in  the  nineteenth  century,  but  its 
relation  to  the  yeasts  was  not  determined  until  1837,  when  Schwann 
demonstrated  that  fermentation  would  not  occur  except  under  the 
influence  of  yeasts.  The  conclusion  that  it  was  the  result  of  the 
growth  of  yeasts  was  vigorously  combated  for  years  by  Liebig,  who 


28         THE    NATURE    OF    THE    ACTIVITIES    OF    MICROORGANISMS. 

looked  upon  the  process  as  a  purely  chemical  change;  but  event- 
ually Pasteur  and  others  proved  that  fermentation  is  a  physiological 
process,  brought  about  only  by  the  growth  of  yeast. 

For  a  long  time  there  was  no  conception  of  more  than  one  type  of 
fermentation.  But  even  in  the  days  of  Schwann  it  was  recognized 
that  there  was  another  type  of  chemical  changes  which  resembled 
the  yeast  fermentation  in  some  respects.  This  was  the  sort  of 
changes  which  occur  in  the  digestion  of  food  and  which  were  known 
even  in  those  early  days  to  be  due  to  certain  materials  present  in  the 
digestive  fluids.  As  early  as  1833  a  substance  called  diastase  was 
known  which  could  convert  starch  into  sugar,  and  in  1836  pepsin, 
causing  the  digestion  of  proteids  in  the  stomach,  was  discovered. 
Although  these  processes  were  realized  to  be  different  from  the 
fermentation  produced  by  yeast,  their  general  similarity  led  to  their 
being  called  fermentations,  and  the  active  substance  in  each  case 
was  known  as  a  ferment. 

It  very  soon  appeared  that  these  two  types  of  fermentation  were 
different  in  some  fundamental  respects.  Whereas  alcoholic  fermen- 
tations, produced  by  yeast,  can  be  stopped  by  certain  chemicals 
like  glycerine,  the  other  type  of  fermentation,  due  to  digestive  fer- 
ments, cannot  be  stopped  by  such  materials.  Moreover,  the 
microscope  shows  that  the  second  type  of  ferments  does  not  contain 
any  living  bodies  like  yeast.  Hence,  while  yeast  is  a  living  ferment, 
the  digestive  ferment  cannot  be  regarded  as  hving.  But  these  latter 
ferments  contain  some  substances  which  are  very  peculiar  in  their 
nature.  Like  living  organisms,  they  are  destroyed  by  high  heat,  and 
they  act  only  at  a  moderate  temperature.  Unlike  most  simple 
chemical  changes,  these  fermentations  do  not  occur  at  high  tempera- 
tures, but  become  impaired  and  stopped  when  the  temperature 
rises  slightly  above  100°  F.  It  has  been  found  possible  to  isolate 
from  the  fermenting  material  (saliva,  gastric  juice,  etc.)  the  ferment- 
ing body.  From  the  digestive  juices  a  substance  can  be  obtained  in 
the  form  of  a  powder  which  can  be  preserved  indefinitely.  It  con- 
tains no  living  cells,  is  not  alive,  and  clearly  does  not  belong  to  the 
same  class  of  bodies  with  the  yeast  plant.  But  it  will  cause  the 
fermentation  to  take  place  when  added  to  a  fermentable  substance. 


IMPORTANCE    OF    FERMENTATION    IN    FARM    LIFE.  29 

These  discoveries  led  to  a  sharp  separation  of  ferments  into  two 
different  classes.  On  the  one  hand  were  those  which,  like  yeast, 
were  produced  by  organisms  and  were  called  organized  ferments, 
and  on  the  other  were  those  which  contained  no  organisms  and 
were  called  unorganized  ferments.  These  latter  ferments  received 
the  name  of  enzymes,  which  name  is  now  in  most  common  use. 

What  are  enzymes?  Over  this  question  there  has  been  not  a 
little  discussion.  But  in  spite  of  it  we  know  very  little  about  them. 
They  seem  to  be  chemical  bodies,  capable  of  producing  chemical 
changes  in  certain  substances.  But  their  action  seems  to  differ 
from  chemical  actions  in  general  in  that  the  ferment  itself  is  ap- 
parently not  used  up  in  the  process.  Whether  this  is  strictly  true 
may,  from  theoretical  reasons,  be  doubted;  but  at  all  events,  no 
direct  evidence  exists  that  they  are  used  up,  and  everything  indi- 
cates that  they  can  act  indefinitely.  A  very  small  amount  of  an 
enzyme  may  produce  a  very  large  amount  of  chemical  change,  and 
the  enzyme  does  not  appear  to  enter  into  the  new  chemical  bodies  in 
any  degree  whatever.  In  some  respects  the  enzymes  resemble 
living  bodies,  especially  in  their  relation  to  heat,  and  in  the  fact  that 
they  are  always  produced  by  living  organisms.  But  in  other  re- 
spects they  are  sharply  marked  off  from  the  organized  ferments.  A 
long  series  of  disinfectants  like  glycerine  and  alcohol,  which  kill 
the  organized  ferments,  have  no  influence  upon  the  enzymes.  The 
latter  do  not  increase  by  growth  in  the  fermenting  material,  nor 
does  their  continued  action  depend  upon  their  nutrition.  The 
opposite  is  true  of  yeast  and  bacteria.  The  distinction  between 
these  two  classes  of  fermentations  has  been  kept  clearly  in  mind 
in  the  development  of  our  knowledge  of  fermentation  in  the  last 
half-century. 

IMPORTANCE  OF  FERMENTATION  IN  FARM  LIFE. 

The  value  of  all  these  types  of  fermentation  in  agriculture  is 
evident  from  the  following  list  of  the  most  important  of  them.  In 
the  first  class  may  be  mentioned: 

The  alcoholic  fermentation;  the  butyric  fermentation,  which  pro- 


30        THE    NATURE    OF   THE    ACTIVITIES    OF    MICROORGANISMS. 

duces  butyric  acid  in  butter;  the  lactic  fermentation,  which  often 
causes  the  souring  of  milk  and  various  other  products,  and  which  is 
responsible  for  the  ripening  of  cream;  the  acetic  fermentation,  which 
produces  acetic  acid  and  forms  vinegar;  the  proteolytic  or  peptonizing 
fermentation,  which  renders  soluble  certain  insoluble  proteids,  an 
example  of  which  is  found  in  the  ripening  of  cheese;  the  oxidizing 
fermentation,  which  causes  the  oxidation  of  organic  matter,  as  in 
the  fermentation  of  tobacco;  the  nitrifying  fermentation,  which  con- 
verts ammonia  into  nitrates  or  nitrites;  the  denitrifying  fermentation, 
which  converts  nitrates  into  nitrites  or  simpler  compounds  by 
depriving  them  of  oxygen.  Then  there  are  the  phenomena  of  putre- 
faction and  decay,  which  are  endless  in  variety  and  which  lie  at  the 
bottom  of  continued  soil  fertility. 

There  is  a  much  longer  list  of  the  unorganized  ferments,  or 
enzymes,  derived  from  both  plants  and  animals.  Some  of  the 
most  important  are  the  following : 

Diastase,  found  in  both  plants  and  animals,  which  changes 
starch  to  sugar;  inulase,  which  has  a  similar  action  upon  inulin; 
invertase,  trehalase,  rafinase,  melizitase,  lactase,  which  act  upon 
sugars,  changing  their  chemical  formula?,  or,  as  the  chemist  says, 
inverting  them.  Emulsin,  myrosin,  erythrozym,  tannase,  lotase, 
and  some  others,  which  act  upon  chemical  substances  called  gluco- 
sids;  pepsin,  trypsin,  from  animal  digestive  juices,  and  galactase 
from  milk,  together  with  hromelin,  papain,  and  vegetable  trypsin, 
from  plants,  which  act  upon  proteids,  causing  them  to  change 
into  simpler  compounds.  These  are  called  proteolytic  enzymes. 
Lipase  acts  upon  fats,  splitting  their  chemical  molecules;  rennet 
acts  upon  milk,  causing  it  to  curdle;  thromhase  is  in  the  blood  and 
is  the  immediate  cause  of  blood  clotting;  cytase  is  an  important 
enzyme,  acting  upon  several  parts  of  a  plant  cell  and  causing  the 
cell  structure  to  disintegrate;  pectase  causes  the  formation  of 
vegetables  jellies  from  materials  in  vegetable  cells;  urase  brings 
about  the  ammoniacal  fermentation  of  urea.  All  of'  the  actions 
above  mentioned  are  the  result  of  chemical  decomposition  in  the 
fermenting  body,  generally  accompanied  by  the  absorption  of  water. 
There  is  another  class  of  enzymes,  called  oxidases,  which  cause 


ORGANIZED   AND    UNORGANIZED    FERMENTS.  31 

the  fermenting  body  to  absorb  oxygen.  Among  these  are,  laccase, 
an  enzyme  concerned  in  the  formation  of  lacquer  varnish  from  sap; 
tyrosinase,  producing  colors  in  fungi;  and  oenoxydase,  an  enzyme 
that  causes  certain  diseases  in  wine.  This  list  could  be  largely  in- 
creased by  adding  other  less  important  enzymes. 

The  simple  enumeration  of  these  lists  is  sufficient  to  emphasize 
their  variety,  and  only  a  brief  examination  is  needed  to  show  their 
intimate  relation  to  farm  processes.  Nearly  all  of  those  enumerated 
have  a  more  or  less  important  relation  to  farm  life,  and  not  a  few 
farm  products  are  quite  dependent  upon  them,  for  example:  vinegar- 
making,  due  to  the  action  of  both  yeasts  and  bacteria,  and  cheese 
making,  due  to  the  enzyme,  rennet,  and  likewise  to  bacteria. 
When  to  this  list  we  add  the  many  serious  animal  and  plant  diseases 
caused  by  germ  life,  with  which  the  farmer  is  waging  constant  war- 
fare, it  becomes  evident  that  agriculture  and  bacteriology  must 
hereafter  be  closely  combined. 

Recognizing  the  great  variety  of  these  allied  phenomena,  it 
becomes  a  little  uncertain  to  what  the  term  fermentation  should  be 
applied.  It  originally  referred  to  the  alcoholic  fermentation,  but 
later  it  was  applied  to  the  changes  due  to  enzymes,  and  enzymes  as 
well  as  yeasts  were  said  to  be  ferments.  Frequently  it  has  been 
applied  to  any  type  of  sugar  fermentation  brought  about  by  yeasts 
or  bacteria,  by  which  gas  is  produced;  and  when  bacteriologists 
use  the  term  they  usually  refer  simply  to  this  change  in  sugars. 
A  term  with  so  varied  a  meaning  is  of  little  value,  and  to-day  there 
is  a  tendency  to  give  up  its  use,  except  in  a  popular  sense  to  cover 
such  a  general  list  of  phenomena. 

IS  THERE  ANY  DISTINCTION  BETWEEN  ORGAN- 
IZED AND  UNORGANIZED  FERMENTS? 

The  confusion  is  rendered  still  greater  by  the  discovery  of  a  series 
of  facts  that  lead  to  the  breaking  down  of  the  distinction  between 
the  organized  ferments  and  the  enzymes.  It  will  be  noticed  that 
these  enzymes  all  come  from  living  organisms,  being  secreted  by 
them;  pepsin  is  secreted  by  the  gastric  glands,  diastase  by  cells  of 


32         THE    NATURE    OF    THE    ACTIVITIES    OF    MICROORGANISMS. 

the  grain,  etc.  The  larger  part  of  the  enzymes  listed  above  are 
secreted  by  certain  plants.  The  power  to  secrete  enzymes  is  thus 
quite  a  common  property  of  plant  cells.  Indeed,  it  is  becoming 
evident  that  many  so-called  life  processes  are  produced  directly 
by  enzymes  secreted  by  animals  and  plants.  Now,  the  action 
produced  by  the  enzyme  trypsin,  secreted  by  the  digestive  glands  of 
animals,  is  very  similar,  if  not  identical,  with  the  action  produced 
by  certain  of  the  bacteria  when  growing  and  acting  upon  proteid 
food.  It  is  a  natural  question  to  ask  if  it  may  not  be  true  that 
the  bacteria  secrete  an  enzyme  similar  to  trypsin,  and  that  their 
action  upon  their  food  is  really  a  digestion  due  to  the  enzyme  which 
they  secrete.  Are  not  both  cases  properly  called  digestion?  If 
we  can  find  such  an  enzyme  in  a  solution  where  these  bacteria  have 
been  growing  for  a  time,  it  would  follow  that  they  must  have  secreted 
it  and  that  their  action  upon  the  proteid  food  is  due  directly  to  the 
enzyme.  This  question  will  at  once  broaden  into  a  second  one, 
and  we  shall  be  forced  to  ask  whether  the  action  on  all  organized 
ferments  may  not  be  explained  by  supposing  the  living  bacteria 
or  yeasts  to  secrete  an  enzyme  whose  direct  action  is  responsible 
for  the  fermentative  change.  If  this  be  the  case,  the  distinction 
between  the  organized  and  unorganized  ferments  disappears. 
The  so-called  organized  ferments  would  then  act  in  exactly  the 
same  way  as  the  unorganized,  the  difference  being  simply  that 
in  the  one  case  the  enzyme  is  secreted  by  the  active  cells  of  larger 
animals  and  plants,  and  in  the  other  by  the  active  cells  of  bacteria 
and  yeasts. 

Now  this  conclusion  is  not  simply  a  theoretical  one,  but  it  has 
been  demonstrated  to  be  true  for  at  least  a  considerable  portion  of 
the  organized  fermentations.  In  the  first  place,  it  has  been  shown 
that  the  power  of  secreting  enzymes  is  a  common  one  among  fungi; 
common  molds  are  known  to  secrete  enzymes  of  much  the  same 
nature  as  digestive  enzymes.  They  soften  up  proteid  substances, 
in  order,  apparently,  that  they  may  absorb  them.  In  other  words, 
they  "digest"  them  for  their  own  use.  When,  in  pursuance  of  this 
idea,  we  study  carefully  the  various  fermentations  at  first  regarded 
as  belonging  to  the  class  of  organized  ferments,  we  find,  in  some  cases. 


ORGANIZED    AND    UNORGANIZED    FERMENTS.  33 

that  the  living  bacterial  cells  do  secrete  an  enzyme  that  actually 
produces  the  chemical  change  in  the  fermented  body.  For  example; 
there  is  a  class  of  bacteria  that  has  the  power  of  curdling  milk 
without  rendering  it  acid,  an  action  very  similar  to  that  of  the 
enzyme  rennin  (rennet),  secreted  by  the  stomach  glands  of  calves. 
But  since  the  curdling  of  milk  by  bacteria  was  produced  by  living 
organisms  that  grow  and  multiply  during  the  process,  it  was  regarded 
as  one  of  the  class  of  organized  fermentations,  and  was  so  identified. 
But  it  has  been  demonstrated  that  this  curdling  is  due  to  an  enzyme 
secreted  by  the  bacteria,  and  that  this  enzyme  is  quite  similar  to 
rennet.  It  may  be  entirely  separated  from  the  bacteria  cells  and 
preserved  in  the  form  of  a  powder,  somewhat  in  the  same  way 
that  rennet  can  be  separated  from  the  stomach  of  a  young  calf.  It 
will  curdle  milk  as  quickly  as  the  rennet.  Further,  these  same 
bacteria  produce  a  second  enzyme  which  has  the  power  of  digesting 
the  curdled  milk,  and  this  second  ferment  is  similar  to  that  secreted 
by  the  pancreas  of  a  mammal. 

Many  other  examples  of  the  same  nature  might  be  mentioned. 
The  general  processes  of  putrefaction  and  decay  are  produced,  it  is 
true,  by  the  destructive  agency  of  microorganisms,  but  directly,  to  a 
great  extent  at  least,  by  the  enzymes  secreted  by  the  bacteria.  But 
while  many  of  the  organized  fermentations  are  thus  explained, 
some  have  not  been  brought  so  easily  into  this  category,  since  it  has 
been  difficult  to  prove  that  they  do  really  produce  an  enzyme.  The 
longest  known  fermentation  of  all,  the  alcoholic  fermentation  of 
sugar  by  yeast,  did  not  for  a  long  time  disclose  any  enzyme,  even 
though  careful  search  was  made  for  one.  But,  thinking  that  per- 
haps in  this  case  the  yeast  cell  produced  the  enzyme  but  did 
not  excrete  it,  retaining  it  in  its  own  body,  Buchner  devised  a 
method  of  crushing  the  yeast  cell  and  squeezing  out  the  inclosed 
juice.  Upon  doing  this  he  obtained  a  liquid  containing  no  living 
matter,  but  capable  of  producing  the  alcoholic  fermentation  in  a 
normal  manner.  The  liquid  evidently  contained  an  enzyme  which 
had  thus  been  pressed  out  of  the  yeast  cell.  This  enzyme  has  been 
named  zymase.  It  would  seem  from  this  that  the  yeast  cell  is  a 
little  chemical  laboratory  that  manufactures  an  enzyme  and  then 


34        THE    NATURE    OF    THE    ACTIVITIES    OF    MICROORGANISMS. 

takes  inside  of  itself  the  sugar  which  the  enzyme  ferments,  after 
which  the  cell  ejects  the  products  of  fermentation,  alcohol,  and  car- 
bon dioxid. 

There  are  still,  however,  some  fermentations  concerning  which  it 
has  been  impossible  as  yet  to  prove  the  formation  of  an  enzyme. 
The  lactic  acid  bacteria  have  the  power  of  fermenting  milk-sugar 
and  producing  lactic  acid  from  it.  Careful  search  has  been  made, 
for  an  enzyme  with  but  partial  success.  It  is  very  probable  that  here, 
too,  the  enzyme  may  be  produced  and  that  it  is  not  secreted  from 
the  bacterial  cell.  Should  this  eventually  prove  to  be  true,  it 
would  apparently  reduce  all  types  of  fermentation  to  the  one  of 
enzyme  action.  This  would  not  reduce  in  the  slightest  degree  the 
importance  of  the  microorganisms  in  the  matter.  It  would  still  be 
the  fact  that  this  large  class  of  chemical  changes  is  brought  about  by 
the  life  activities  of  living  organisms,  but  we  would  understand  that 
they  perform  their  action  by  first  secreting  enzymes  and  that  the 
enzymes  are  the  direct  agents  for  bringing  about  the  fermentative 
changes. 

It  is  desirable  to  notice  also  that  even  if  we*  accept  the  enzyme 
conception  of  fermentations  we  are  no  nearer  a  satisfactory  under- 
standing of  the  real  nature  of  the  phenomenon.  For  over  fifty 
years  science  has  been  trying  to  explain  these  mysterious  changes 
in  fermentable  bodies.  At  one  time  it  was  thought  that  they  were 
purely  chemical  processes;  but  this  has  been  disproved  by  showing 
that  living  organisms  are  necessary  to  their  production.  Pasteur 
thought  they  were  due  to  "life  without  oxygen,"  claiming  that  the 
living  germ  required  oxygen  for  its  life,  that  if  it  did  not  find  plenty 
of  free  oxygen,  it  would  take  atoms  of  this  element  out  of  the  sugar 
molecules  or  other  fermentable  body,  and  that  the  withdrawal  of  this 
oxygen  caused  the  molecule  to  fall  to  pieces.  This  theory  has  also 
been  abandoned.  The  theory  that  all  living  fermenting  agents  se- 
crete a  chemical  enzyme  appears  to  stand  the  test  of  experiment,  but 
it  explains  little,  for  we  do  not  know  what  enzymes  are  and  we  have 
absolutely  no  knowledge  of  how  they  act.  Are  they  wholly  lifeless 
chemical  bodies  or  are  they  semiliving,  as  some  would  say  ?  What- 
ever they  are,  they  are  still  as  great  mysteries  as  the  fermentations 


FERMENTATIONS    NOT    ALL    DUE    TO    MICROORGANISMS.  35 

they  produce  have  always  been.     The  real  nature  of  the  fermenting 
processes  is,  in  short,  quite  unknown. 

FERMENTATIONS  NOT  ALL  DUE  TO  MICRO- 
ORGANISMS. 

One  final  question  needs  to  be  raised.  Are  all  of  the  many  kinds 
of  slow  progressive  changes,  resembling  fermentations  in  a  broad 
sense,  due  to  the  action  of  microorganisms,  either  by  direct  action 
or  through  the  agency  of  the  enzymes  they  produce  ?  After  we  have 
recognized  that  higher  plants  may  produce  enzymes,  we  see  at  once 
that  there  may  be  certain  fermentative  processes  in  the  soil  or  else- 
where, for  which  microorganisms  are  not  directly  responsible.  If  a 
plant  produces  an  enzyme,  this  body  may  remain  ready  for  action 
after  the  plant  which  produced  it  is  dead,  and  fermentative  changes 
may  go  on  in  a  mass  of  vegetable  tissue  for  which  microorganisms 
are  not  responsible.  It  very  commonly  happens  that  after  the 
death  of  the  plant  it  undergoes  some  kind  of  fermentative  change. 
When  piled  into  a  compost  heap  or  stored  in  a  silo,  the  plant  tissues 
certainly  show  unquestionable  evidence  of  marked  fermentative 
changes.  These  phenomena  are  accompanied  by  a  rise  in  tempera- 
ture and  have  all  the  characteristics  of  true  fermentation.  In 
such  heaps  bacteria  are  certainly  present  and  the  rapidly  widening 
conception  of  the  agency  of  bacteria  in  producing  fermentations 
led  to  the  conclusion  that  they  cause  all  such  fermentations.  But 
the  growing  knowledge  of  the  nature  and  abundance  of  enzymes 
is  leading  to  the  conclusion  that  some  of  these  fermentations  are  not 
due  to  bacterial  action  at  all,  but  simply  to  the  enzymes  which  were 
excreted  by  the  plants  during  their  life  and  which  get  a  chance  to  act 
in  the  fermenting  heap.  If  the  corn  during  life  produced  enzymes, 
these  would  find  their  way  into  the  silo  and  inevitably  start  fer- 
mentations which  would,  of  course,  have  nothing  to  do  with  bacteria. 

While,  then,  fermentations  and  putrefactions  must,  in  general,  be 
attributed  to  germ  life,  we  must  ever  bear  in  mind  that  similar  or 
identical  phenomena  may  sometimes  be  caused  by  enzymes  from  a 
different  source. 


36         THE    NATURE    OF    THE    ACTIVITIES    OF    MICROORGANISMS. 

THE  PURPOSE  OF  FERMENTATION. 

All  these  types  of  fermentations,  whether  caused  by  the  metabo- 
lism of  bacteria  or  yeasts,  or  by  enzymes  secreted  by  these  organ- 
isms or  by  higher  plants,  are  of  vital  importance  in  agricultural 
processes.  Without  their  agency  in  breaking  up  organic  com- 
pounds, the  soil  would  rapidly  become  unfit  for  supporting  life. 
The  agricultural  industry  is  not  only  dependent  upon  fermentations 
for  many  minor  processes,  but  it  is  fundamentally  dependent  upon 
them  for  its  continuance.  While  this  is  true,  it  must  not  be  as- 
sumed that  the  various  bacteria  produce  their  results  for  the  benefit 
of  agriculture  or  for  the  benefit  of  the  soil.  There  is  no  purpose 
in  the  matter.  Each  species  of  animal  and  plant  acts  its  own  life 
for  its  own  good.  If  it  secretes  an  enzyme  that  produces  a  fermenta- 
tion, this  is  done  for  its  own  benefit  and  not  for  the  farmer's.  The 
yeast  ferments  sugar,  and  bacteria  putrefy  proteids  for  uses  of  their 
own.  Incidentally  it  may  result  that  the  natural  processes  of  life 
phenomena  are  benefited  thereby;  but  primarily  all  of  the  enzymes 
secreted  and  all  of  the  fermentations  produced  are  for  the  benefit  of 
the  organisms  secreting  them.  If  a  bacterium  or  a  mold  secretes 
an  enzyme  into  a  lot  of  milk  which  causes  its  digestion,  its  purpose  is 
to  digest  the  milk  for  its  own  use  and  not  for  any  incidental  results 
that  may  accrue  to  the  cheese-maker. 


PART  II. 

BACTERIA  IN  SOIL  AND  WATER. 


CHAPTER  III. 
NATURE'S  FOOD-SUPPLY.     THE  CARBON  CYCLE. 

THE  CONTINUATION  OF  THE  FOOD-SUPPLY. 

The  farmer's  primary  occupation  consists  in  converting  soil, 
water,  and  air  into  human  food.  This  he  does  through  the  agency 
of  plants  that  grow  in  the  soil  and  furnish  the  food  necessary  for 
his  stock,  in  addition  to  a  part  of  his  own  food.  So  long  as  plants 
find  in  the  soil  proper  conditions  for  growth,  the  food-supply  will  not 
fail.  The  problem  of  keeping  up  the  food-supply  of  plants  thus 
becomes  the  one  problem  of  supreme  importance. 

By  far  the  largest  part  of  the  plant  food,  in  weight,  comes  from 
the  air  in  the  form  of  carbonic  dioxid  and  water,  and  these  two 
substances  are  practically  inexhaustible.  But,  in  addition,  some 
foods  are  obtained  from  the  soil.  These  last  are  present  in  the  soil 
in  limited  quantities  only,  and  some  of  tjiem  are  found  only  in  the 
upper  layers.  They  are  constantly  being  used  by  successive  genera- 
tions of  plants.  This  constant  use,  in  the  course  of  centuries, 
would  have  quite  exhausted  the  soil  were  there  not  some  means  by 
which  these  supplies  were  replaced.  That  there  is  some  such  means 
is  evident  from  the  fact  that  plants  have  continued  to  grow  on 
the  same  soil  for  countless  generations,  the  soil  remaining  as  fertile 
as  ever.  Clearly  the  problem  for  agriculturists  is  to  find  out  the 
factors  that  have  kept  up  the  fertility  of  virgin  soil  and  to  apply 
them  properly  to  cultivated  soil.     In  this  way  only  can  the  continued 

37 


38  nature's  food-supply,     the  carbon  cycle. 

fertility  of  the  soil  be  assured.  While  various  agencies  are  concerned 
in  this  matter  of  soil  fertility,  the  agency  of  microorganisms  is 
certainly  one  of  the  largest. 

PLANT  FOODS. 

The  green  plants  live  chiefly  upon  the  following  foods : 

Water. — This  material,  coming  from  the  rains,  is  unlimited  in 
amount  and  need  not  detain  us. 

Carbonic  Dioxid. — This  gas  (CO^)  furnishes  the  carbon 
which  is  the  basis  of  most  plant  structures,  wood,  cellulose,  starch, 
sugar,  etc.  It  is  present  in  the  air  in  small  percentage  only,  but 
is  kept  fairly  constant  by  processes  which  we  shall  consider. 

Nitrates. — These,  which  are  salts  of  nitric  acid  (HNO3), 
constitute  the  chief  form  in  which  plants  obtain  their  nitrogen. 
Nitrogen  in  considerable  amount  is  an  absolute  necessity  for  all 
plant  life,  and  while  plants  can  probably  assimilate  some  nitrogen 
from  ammonia,  it  is  certain  that  ordinarily  they  do  not  obtain  much 
from  this  source.  The  higher  compounds  of  nitrogen,  like  proteids, 
ures,  or  other  complex  bodies,  cannot  furnish  plants  with  nitrogen 
directly,  nor,  on  the  other  hand,  can  nitrites  (salts  of  nitrous  acids, 
HNO2)  or  free  nitrogen  in  the  air  supply  any  nitrogen  directly  to 
plants.  Practically  all  the  nitrogen  must  be  obtained  by  the  plants 
in  the  form  of  nitrates  from  the  soil,  and  to  keep  a  constant  supply 
of  nitrates  in  the  soil  must  be  the  first  aim  of  the  farmer. 

Phosphates. — A  small  amount  of  phosphorus  is  needed  by 
plants  and  is  obtained  in  the  form  of  the  soluble  phosphates  from 
the  soil.  The  mineral  soil  ingredients  contain  much  phosphorus 
in  insoluble  compounds,  and  agencies  for  rendering  these  soluble 
are  necessary  to  soil  fertility. 

Potash. — Some  form  of  potassium  salts  is  necessary.  These 
salts  abound  in  soils,  but  some  agency  must  be  employed  to  dissolve 
them. 

Sulphates. — These  salts  are  also  needed  in  small  amounts 
only. 

Iron  Salts. — Needed  in  small  quantities  only. 


MICROORGANISMS    IN    THE    SOIL.  39 

Lime  and  magnesiaf  should  also  be  in  the  soil  for  reasons  that 
will  be  given  later,  but  they  are  only  slightly  used  by  most  plants. 

There  are  still  other  materials  used  by  plants  in  very  minute 
quantities,  but  they  hardly  fall  in  the  scope  of  our  study.  All 
the  foods  above  mentioned  are  commonly  called  inorganic  foods, 
since  they  come  chiefly  from  the  soil  and  the  air.  Organic  foods 
on  the  other  hand  refer  to  the  more  highly  organized  products,  which 
are  the  immediate  remains  of  living  things,  like  roots,  starches, 
fats,  wood,  cellulose  and  other  similar  bodies.  ,Our  problem,  then, 
is  to  explain  nature's  methods  of  keeping  the  soil  supplies  of  these 
various  inorganic  ingredients  from  diminishing. 

MICROORGANISMS  IN  THE  SOIL. 

The  upper  layers  of  the  soil  are  exceedingly  rich  in  bacteria, 
the  number  varying  according  to  conditions,  from  a  few  thousands 
to  many  millions  per  gram.  In  sandy  soil  there  may  be  very  few, 
while  in  soil  polluted  with  organic  matter,  as  in  the  vicinity  of 
manure  heaps,  there  may  be  as  many  as  100,000,000  per  gram  or 
even  more,  1,600,000,000  per  gram  having  been  found  in  some  soils. 
They  rapidly  diminish  in  numbers,  as  we  pass  to  the  lower  layers, 
and  at  a  depth  of  from  four  to  six  feet,  they  have  almost  disappeared. 
Below  this  they  are  rarely  found,  except  in  places  where  drainage 
currents  carry  them  downward.  The  microorganisms  thus  found 
in  the  soil  include  bacteria  in  the  greatest  abundance,  and  also 
quantities  of  the  higher  fungi  and  yeasts.  Each  of  these  classes  is 
represented  by  many  varieties,  and  each  has  an  important  share  in 
the  complex  activities  going  on  in  the  soil.  These  functions  and 
the  relation  of  the  soil  microorganisms  to  them  may  best  be  under- 
stood by  noticing  in  succession  their  relation  to  the  various  soil 
ingredients  that  constitute  plant  foods. 

ORIGIN  OF  SOIL. 

The  ingredients  in  the  soil  may  be  divided  into  two  classes:  i. 
The  purely  mineral  matters.  2.  The  organic  ingredients  consti- 
tuting the  humus. 


40  NATURE  S    FOOD-SUPPLY.       THE    CARBON    CYCLE. 

The  Mineral  Ingredients. — These  come  primarily  from  the 
rocks  that  constitute  the  earth's  surface,  soil  being  sometimes  de- 
scribed as  ground-up  rock.  The  agents  that  cause  the  grinding  of 
the  rocks  are  physical,  chemical,  and  biological.  The  physical 
agencies  are  chiefly  those  of  freezing  and  thawing,  together  with  the 
solvent  action  of  waters.  The  chief  chemical  agent  is  direct  oxida- 
tion by  the  oxygen  of  the  atmosphere.  The  physical  and  chemical 
agents  together  produce  what  has  been  called  the  "weathering" 
of  rocks,  resulting  in  their  crumbling  into  fine  fragments  With  these 
we  are  not  particularly  concerned.  The  biological  agencies  are  those 
of  the  soil  microorganisms.  We  do  not  yet  know  very  definitely  how 
great  a  part  they  play  in  this  process,  but  that  it  is  an  important  part 
is  surely  proved.  One  of  the  results  of  their  growth  is  the  liberation 
of  carbonic  dioxid  from  decomposing  masses.  This  gas  is  readily 
dissolved  in  the  soil  water,  and  water  containing  carbonic  dioxid  in 
solution  is  able  to  dissolve  a  considerable  quantity  of  carbonate  of 
lime.  These  carbonated  waters,  therefore,  play  a  great  part  in  the 
disintegration  of  limestone,  which  is  one  of  the  prominent  factors 
concerned  in  the  formation  of  soils.  Again,  the  microorganisms 
which  decompose  organic  matters  in  the  soil  produce  a  variety  of 
organic  acids.  Among  these  are  the  lactic,  butyric,  and  acetic  acids, 
as  well  as  many  others.  These  acids  have  a  solvent  action  upon 
various  rock  formations,  and,  by  dissolving  out  certain  parts  of  the 
rocks,  they  slowly  but  surely  cause  them  to  crumble.  Some  of 
these  matters  will  be  considered  on  later  pages  in  other  connections, 
but  we  are  interested  in  them  here  as  showing  that  bacteria  are 
prominently  concerned  in  the  disintegrations  of  rocks  which  result 
in  the  formation  of  soil. 

The  Humus. — There  is  a  vast  difference  in  the  fertility  of  a 
sand  and  a  garden  soil,  Sandy  soil  may  contain  all  the  necessary 
mineral  matters,  but  it  lacks  the  something  needed  for  plant  growth 
which  the  garden  soil  contains.  This  something  is  called  humus, 
an  element  rather  difficult  to  define  and  still  more  difficult  to  describe 
in  chemical  terms.  It  is  abundant  in  fertile  soil,  but  scarce  or 
wanting  in  barren  soil.  Though  its  chemical  value  is  too  complex 
to  be  stated  or  even  known,  its  origin  is  easy  to  understand. 


THE  TRANSFORMATION  OF  CARBON.  41 

Humus  is  the  remains  of  life  of  previous  generations.  When 
plants  die,  their  roots,  together  with  their  leaves,  branches,  and 
fruits,  inevitably  become  incorporated  into  the  soil.  Animals,  too, 
leave  upon  the  ground  a  quantity  of  excrement  and  other  discharges ; 
and  plants  likewise  probably  discharge  excretions  into  the  soil.  When 
animals  die  their  bodies  also  may  become  mixed  with  the  earth. 
Thus,  practically  all  kinds  of  organic  matter  from  animals  and 
plants  are  being  mixed  continually  with  mineral  ingredients  in  the 
surface  layers  of  the  soil.  The  microorganisms  in  the  soil  feed 
upon  these  dead  materials,  causing  an  extensive  series  of  decomposi- 
tions and  recombinations.  To  this  mass  of  complex  organic  bodies 
undergoing  decomposition  in  the  soil  has  been  given  the  name 
humus.  It  will  be  evident  from  this  explanation  of  its  origin  that 
humus  cannot  have  a  definite  composition,  and  that  it  will  hardly  be 
alike  in  any  two  soils.  It  will  be  composed  of  different  materials 
to  start  with,  and  there  will  be  a  variety  of  different  stages  of  decom- 
position. W^e  cannot  hope  to  find  any  definite  composition  of 
humus,  but  we  can  study  the  kinds  of  decomposition  and  recombina- 
tions that  are  going  on  in  it  and  that  result  in  making  it  a  suitable 
food  for  plants.  In  this  study  we  must  ever  keep  in  mind  the  fact 
that  dead  bodies  of  animals  and  plants  are  not  in  condition  to  serve 
another  generation  of  plants  as  food.  We  cannot  feed  plants  upon 
eggs,  or  urine,  or  starches,  or  sugars.  Though  containing  carbon 
and  nitrogen  in  abundance,  these  elements  are  locked  up  in  them  out 
of  the  reach  of  the  green  plants,  and  before  they  can  be  utilized 
again  they  must  be  freed  from  their  combinations  and  brought  into 
simpler  forms.  This  is  accomplished  by  the  microorganisms  in  the 
soil.  Our  study  of  these  changes  may  best  be  centered  around  the 
two  chemical  elements,  carbon  and  nitrogen. 

THE  TRANSFORMATION  OF  CARBON. 

The  green  plants  seize  the  carbon  dioxid  (COa)  from  the  air  by 

means  of  their  leaves  and,  utilizing  the  energy  of  sunlight,  build  this 

carbon  into  higher  compounds.    Starch  is  formed  first,  and  later  other 

substances — cellulose,  wood,  fats,  sugar — are  built  from  the  elements 

4 


42  NATURE  S    FOOD-SUPPLY.       THE    CARBON    CYCLE. 

found  in  the  starch  alone;  while,  by  combining  them  with  some 
nitrogen,  various  proteid  bodies  are  produced.  When  once  carbon 
has  assumed  these  forms  it  is  no  longer  within  the  reach  of  another 
generation  of  plants.  It  is  locked  up  and  cannot  again  be  utilized 
until  it  has  once  more  been  reduced  to  a  condition  of  carbon  dioxid. 

The  compounds  thus  built  up  have  different  destinies.  Some  of 
these  are  eaten  by  the  animal  kindgom  and,  after  serving  the  needs 
of  the  animals,  are  exhaled  as  CO2  to  join  the  atmosphere  again.  A 
large  part  is  not  appropriated  by  animals,  but  begins  at  once  to 
undergo  destructive  changes  which  bring  their  ingredients  back  again 
to  their  starting-point.  Some  of  the  processes  of  chemical  destruction 
are  comparatively  simple.  The  starches,  sugars,  and  fats  are  sub- 
ject to  chemical  changes  which  take  place  under  the  direct  in- 
fluence of  chemical  forces,  since  they  may  be  directly  oxidized.  All 
forms  of  active  combustion  in  fires  produce  such  oxidation,  the  result 
of  which  is  that  the  carbon  in  the  compounds  burned  is  united  with 
oxygen  and  liberated  in  the  form  of  CO2,  the  hydrogen  being 
liberated  in  the  form  of  water.  These  join  the  atmosphere,  while 
the  minerals  remain  behind  as  ash.  Thus,  all  forms  of  combustion 
in  carbonaceous  material  restore  some  of  the  carbon  to  the  atmos- 
phere in  the  form  of  CO2,  and  upon  this  the  plants  again  feed. 

But  although  direct  oxidation  may  form  a  considerable  part  of 
this  process  of  food  reduction,  another  very  large  factor  is  due  to  the 
agency  of  microorganisms.  Fires  rarely  occur  in  nature,  unless 
started  by  man,  and  there  must  be  some  other  means  of  oxidation. 
A  slow  oxidation  of  carbonaceous  material  occurs  in  nature  at  all 
times,  and  ordinarily  it  has  been  attributed  to  direct  chemical 
processes.  It  is  quite  doubtful,  however,  if  this  slow  oxidation 
would  occur  were  it  not  for  the  agency  of  microorganisms.  At  all 
events  a  considerable  part  of  the  so-called  slow  oxidizing  processes 
is  the  direct  result  of  their  growth.  The  various  kinds  of  organisms 
bring  about  the  gradual  destruction  of  the  different  types  of  car- 
bonaceous materials. 

Sugars. — These  are  contained  in  fruits  and  some  vegetables, 
and  as  they  decay,  the  sugar  commonly  undergoes  an  alcoholic 
fermentation,  produced  by  the  action  of  yeasts  and  molds.     The 


THE    TRANSFORMATION    OF    CARBON.  43 

fermentation  which  goes  on  in  a  decaying  apple  is  identical 
with  that  which  occurs  in  the  brewer's  vat.  The  result  is  the 
formation  of  CO2  and  alcohol,  the  carbon  dioxid  passing  into 
the  atmosphere  to  contribute  to  the  store  of  this  important  food. 
The  alcohol,  under  normal  conditions,  also  passes  into  the  air  and 
is  eventually  further  oxidized  into  carbonic  acid  and  water.  Thus, 
the  carbon  of  the  sugars,  by  the  agency  of  yeasts  and  molds,  is 
restored  to  the  air.  Starches  have  nearly  the  same  history,  since 
they  are  readily  converted  into  sugars  by  enzymes  secreted  by  the 
plants,  and  are  then  fermented.  To  a  certain  extent  bacteria 
also  ferment  sugars,  producing  a  series  of  acids. 

Cellulose. — Cellulose  is  a  material  closely  related  to  starch,  and 
is  found  in  the  cell  walls  of  all  plants.  Wood  and  straw  contain  it 
in  considerable  quantity,  while  cotton  and  wood  fibers  are  almost 
pure  cellulose.  Swedish  filter-paper  is  one  of  the  purest  forms. 
The  material  is  quite  resistant  to  ordinary  forms  of  decay  and  is 
seldom  affected  by  common  plant  decay.  But  certain  bacteria 
are  able  to  act  upon  it  so  as  to  ferment  it  and  set  free  its  carbon. 
Several  of  these  have  been  isolated  and  studied.  Some  of  them  act 
in  the  absence  of  oxygen  (anaerobic),  while  others  act  only  in  its 
presence  (aerobic).  The  former  can  carry  on  their  activity  in 
the  midst  of  a  manure  heap  so  tightly  packed  as  to  exclude  air, 
while  the  latter  will  occur  in  moistened  masses  of  vegetable  tissue 
exposed  to  the  air.  These  cellulose-fermenting  bacteria  are  abun- 
dant everywhere  and  are  constantly  at  work  in  the  soil,  fermenting 
the  hard  cellulose  parts  of  the  great  variety  of  plant  roots,  stems,  and 
leaves  that  accumulate  in  the  soil  or  in  the  waters  of  streams  and 
swamps.  When  the  mass  is  alkaline  in  reaction  the  cellulose  may  be 
fermented  by  bacteria;  but  when  it  is  acid,  as  in  sour  soils,  bacteria 
cannot  grow.  Under  these  conditions  certain  of  the  molds  in  the 
soil  may  ferment  the  cellulose.  The  chemical  nature  of  the  fermen- 
tation need  not  concern  us,  only  so  far  as  to  notice  that,  as  a  result, 
the  carbon  is  set  free,  either  in  the  form  of  carbonic  dioxid  or 
marsh  gas  (CHJ,  the  latter  gas  becoming  readily  converted  later 
into  carbon  dioxid.  The  total  result  is  the  restoration  of  the 
carbon  to  its  original  condition  in  the  air,  where  it  can  be  utilized 


44  nature's  food-supply,     the  carbon  cycle. 

by  the  next  generation  of  plants.  Certain  mineral  matters  are 
also  set  free  from  the  cellulose  in  the  form  of  ash,  which  adds  to  the 
fertility  of  the  soil. 

A  fermentation  of  cellulose  is  believed  to  occur  also  in  the  intes- 
tines of  herbivorous  animals.  These  animals  utilize,  to  a  certain 
extent,  cellulose  materials  as  a  food;  these  undergo  a  fermentation 
in  the  intestines  resulting  in  the  formation  of  certain  substances 
that  are  assimilated  by  the  animals  as  food.  Cellulose-fermenting 
bacteria  are  found  in  the  intestines  of  such  animals  in  considerable 
abundance,  and  it  is  thought  that  they  play  an  important  part  in 
the  ordinary  digestion  of  celluloses.  Whether  the  animal  might 
not  be  able  to  digest  them  without  the  aid  of  bacteria  has  not  yet 
been  proved,  but  it  is  almost  certain  that  the  bacteria  do,  under 
ordinary  conditions,  play  an  important  part  in  the  process.  The 
fermentation  begun  in  the  intestines  is  finally  completed  in  the 
manure  heap,  and  thus,  after  a  time,  the  cellulose  is  completely  de- 
composed and  its  carbon  restored  to  the  atmosphere. 

Wood. — Another  product  of  plant  life  somewhat  closely 
related  to  cellulose  is  woody  tissue.  The  fermentation  and  destruc- 
tion of  wood  is  certainly  a  matter  of  necessity,  if  the  carbon  supply 
is  to  be  kept  constant.  That  there  is  such  a  fermentation  is  evident 
to  anyone  who  has  walked  through  a  forest  and  noticed  the 
condition  of  the  fallen  trunks  and  branches.  A  fallen  tree  will 
remain  for  a  time  upon  the  surface  of  the  ground,  apparently 
unaltered.  But  presently  it  becomes  softened  by  some  agency,  not 
manifest  at  first,  and  the  hard,  woody  mass  is  slowly  but  surely 
converted  into  a  soft  friable  substance,  which  eventually  crumbles 
into  a  brownish  powder  and  is  incorporated  into  the  soil,  contributing 
to  the  formation  of  the  humus.  This  destruction  of  woody  tissue 
is  also  brought  about  by  microorganisms,  but  in  this  case  it  is  not 
bacteria  that  are  at  first  concerned. 

The  first  phenomenon  that  occurs  in  such  a  decaying  tree  trunk 
is  the  growth  of  larger  fungi.  Various  forms  of  mushrooms  and 
tree  fungi  start  their  growth  on  its  surface  and  send  delicate  mycelium 
threads  into  the  substance  of  the  wood.  These  threads  grow  first 
underneath  the  bark  and  in  the  superficial  layers  of  wood;  but 


THE    TRANSFORMATION    OF    CARBON. 


45 


gradually  they  penetrate  the  hard  wood  and,  by  the  chemical 
excretions  they  produce,  soften  this  hard,  tough  substance.  With- 
out the  growth  of  such  fungi  in  the  wood  there  would  seem  to 
be  no  way  of  softening  the  wood  sufficiently  for  decay.  After  the 
wood  has  been  somewhat  softened  by  the  fungi,  wood-eating  in- 
insects  begin  their  work  upon  it,  using  the  fungi  largely  as  food. 
It   is   probable  that  bacteria  also  may  assist  in  this  matter,  but 


BACTERIA 


VARIOUS  CARBON 
COMPOUNDS  BUT 
EVENTyUALLY  ALL 
REDUCED  TO   CO2 


vT  /f 

IN  ATMOSPHERE 

Fig.  13. — The  carbon  cycle. 


the  larger  fungi  are  chiefly  responsible  for  the  destruction  of  the 
woody  tissue.*  The  final  result  is  that  the  carbonaceous  material 
in  the  wood  is  liberated  by  being  combined  with  oxygen,  and  passes 
off  into  the  air  to  join  the  atmospheric  store  of  carbon.  The 
hydrogen  and  oxygen  are  converted  into  water,  and  in  their  turn 
enter  the  atmosphere  as  water  vapor.     In  this  way,  by  a  slow  process 

♦These  same  processes,  so  useful  in  the  general  changes  in  nature,  are. 
of  decided  disadvantage  when  they  occur  in  timber  that  is  desirable  to 
be  preserve  1.  The  ordinary  decay  of  timber  is  brought  about  by  the 
kind  of  fungi  and  bacteria  above  mentioned.  Since  none  of  these  or- 
ganisms can  grow  without  water,  it  follows  that  well-dried  wood  will  not 
decay,  from  which  is  to  be  drawn  the  lesson  that  the  best  method  of  pre- 
serving timber  is  by  thorough  drying. 


46  nature's  food-supply,     the  carbon  cycle. 

of  decomposition,  wood  is  converted  into   simple  chemical  com- 
pounds which  join  nature's  food-supply  in  the  air. 

By  the  means  thus  indicated  the  large  part  of  the  carbon  ex- 
tracted from  the  air  by  plants  is  restored  again  to  the  air  in  the  form 
in  which  it  first  existed.  This  carbon  cycle  is  represented  graphi- 
cally in  the  accompanying  diagram  (Fig.  13). 


CHAPTER  IV. 

NITROGEN.     DECOMPOSITION  OF  NITROGENOUS 
COMPOUNDS. 

The  nitrogenous  foods  of  plants  are  next  in  importance  to  carbon 
dioxid  and  water.  Plants  cannot  grow  without  nitrogen,  and  they 
need  it  in  larger  quantity  than  any  other  mineral  foods.  The  ni- 
trogenous fertilizers  have  commonly  a  more  noticeable  effect  in 
stimulating  crops  than  other  minerals.  The  amount  of  material 
in  the  world  that  can  serve  directly  as  nitrogen  food  for  plants  is 
decidedly  limited,  and  therefore  it  is  expensive.  For  these  as  well 
as  other  reasons,  the  problem  of  continued  soil  fertility  is  more 
closely  bound  up  with  the  matter  of  nitrogen  than  any  other  chemical 
element. 

SOURCES  OF  NIROGENOUS  FOOD. 

Plants  take  their  nitrogen  from  the  soil,  chiefly  in  the  form 
of  nitrates.  While  it  is  true  that  they  can  utilize  ammonium 
salts  also,  under  ordinary  conditions  ammonia  furnishes  little 
food  directly  to  the  plant,  the  far  larger  part  being  furnished 
by  soil  nitrates.  The  amount  of  nitrate  in  any  soil  is  however, 
very  limited,  there  being  only  from  o.i  per  cent,  to  0.2  per  cent, 
in  ordinary  soils.  As  crop  after  crop  is  grown,  the  small  amount  in 
the  soil  is  gradually  used  up  and  must  be  replaced  if  the  soil  is  to 
continue  yielding  crops.  The  farmer  buys  nitrates  in  the  form  of 
commercial  fertilizers  to  replace  the  amount  taken  from  his  soils  by 
his  crops.  These  commercial  nitrates,  however,  are  also  limited 
in  amount.  They  are  confined  to  a  few  deposits  of  nitrates,  chiefly 
in  warm  dry  regions.  The  best  known  come  from  Chili,  whose 
nitrate  mines  to-day  furnish  the  greater  part  of  the  nitrates  for  the 

47 


48       NITROGEN.       DECOMPOSITION  OF  NITROGENOUS  COMPOUNDS. 

world.  But  these  beds  do  not  solve  the  question  of  continued  soil 
fertility,  for  they  will  soon  be  exhausted  and  some  other  source  of 
nitrates  must  be  discovered.  Fortunately,  we  have  learned  that 
there  are  processes  in  nature  by  which  the  soil  nitrates  are  replaced, 
quite  independently  of  the  store  of  nitrates  in  Chili  or  elsewhere, 
and  this  replacement  is  a  phenomenon  of  the  life  activities  of  the 
bacteria  and  other  microorganisms  of  the  soil. 

If  the  nitrates  absorbed  from  the  soil  are  replaced,  it  must  be 
from  one  of  two  sources  or  from  both. 

1.  From  the  Nitrogen  in  Organic  Bodies. — After  plants  absorb 
nitrates  from  the  soil,  they  build  them  up  into  organic  compounds, 
chiefly  proteids,  which  subsequently  may  or  may  not  be  utilized  as 
food  by  animals.  Whatever  be  its  history,  whether  in  animals  or 
plants,  this  proteid  material  always  contains  nitrogen;  and  eventually, 
by  processes  which  we  shall  study,  this  nitrogen  may  again  assume 
the  form  of  nitrates  and  restock  the  soil. 

2.  The  Nitrogen  of  the  Atmosphere. — This  is  an  inexhaustible 
source  of  supply,  if  it  can  be  utilized.  We  shall  learn  that  there 
are  means  by  which  it  becomes  available  for  plants. 

There  seem  to  be  no  other  possible  sources  for  replacing  soil 
nitrates,  and  we  will  therefore  consider  these  two  in  detail. 

ORGANIC  NITROGEN.     ITS  NATURE. 

The  nitrates  are  built  up  by  the  plants  into  a  variety  of  com- 
pounds, mostly  of  the  nature  of  proteids,  like  gluten  of  wheat, 
legumen  of  peas,  and  other  similar  bodies.  All  plants  contain  some 
such  compounds  which  serve  a  purpose  in  the  life  of  the  plant. 
While  the  plant  is  still  alive  they  remain  as  proteids  without  much 
change.  After  the  death  of  the  plant  that  produced  them  the  pro- 
teids are  at  the  disposal  of  nature's  forces  of  destruction.  Some  of 
the  proteids  are  seized  by  animals  and  utilized  for  their  life  proc- 
esses. They  are  slightly  changed  inside  the  animal's  body,  but  are 
not  built  into  bodies  more  complex  than  proteids.  The  animal 
forms  animal  proteids  out  of  them,  producing  myosin,  gelatin, 
chrondin,  and  other  compounds,  none  of  which  are  likely  to  be  more 


ORGANIC   NITROGEN.       ITS   NATURE.  49 

complicated  than  the  original  proteid  of  the  food.  Thus,  in  the 
bodies  of  plants  and  animals  alike  the  nitrogen  reaches  a  condition 
allied  to  proteid.  But,  while  proteids  may  serve  as  food  for  animals 
and  for  the  great  class  of  colorless  plants  (fungi)  they  are  quite  out 
of  the  reach  of  the  green  plants,  which  are  the  great  food  producers 
of  nature.  Our  next  problem,  then,  must  be  to  learn  how.  these 
proteids  are  reduced  to  their  original  condition  of  nitrate. 

Part  of  the  proteid  thus  built  up  into  the  body  of  the  plant  or  the 
animal  remains  there  until  the  animal  or  plant  dies,  and  at  death  it 
is  still  a  proteid  and  as  complex  as  ever.  In  this  form  it  may  be- 
come incorporated  into  the  soil  when  the  animal  or  plant  dies,  or  it 
may  become  eaten  as  food  and  pass  through  the  body  of  another 
animal.  But  much  of  it  will  eventually  reach  the  soil  while  still 
in  the  form  of  proteid. 

A  second  portion  of  the  proteid  is  used  up  in  the  animal's 
body  to  furnish  energy  and  heat;  it  is  metabolized,  as  we  say. 
When  it  is  thus  used  its  complex  chemical  molecule  is  broken  to 
pieces,  and  it  is  reduced  to  much  simpler  compounds.  But  it  is  not 
decomposed  sufi&ciently  to  bring  the  nitrogen  back  within  the  reach 
of  plant  life.  The  carbon  in  this  proteid  is  in  part  removed  from  it 
and  combined  with  oxygen,  to  be  exhaled  as  CO2.  The  molecule 
falls  to  pieces  and  various  simpler  by-products  arise;  but  in  the 
animal's  body,  practically  all  of  it  eventually  assumes  the  form  of 
urea  (CONaHJ.  Though  this  urea  is  a  nitrogen  molecule  far 
simpler  than  proteid,  still  it  is  not  simple  enough  for  a  plant  food. 
Urea,  or  a  closely  allied  compound,  is  the  form  in  which  nearly  all  of 
the  nitrogenous  material  resulting  from  proteid  metabolism  in  the 
animal  body  is  excreted.  Urea  thus  represents  one  stage  in  the 
destruction  of  proteid  compounds,  and  to  this  stage  the  proteids  are 
brought  as  the  result  of  the  metabolism  in  the  life  processes  of 
animals.  In  some  animals  this  urea  is  secreted  as  urine  by  the 
kidneys,  but  in  others  (birds)  it  is  mixed  with  the  feces;  in  all  cases 
it  contains  the  nitrogen  which  is  no  longer  of  any  use  to  the  animal 
world.  It  is  estimated  that  some  38,000  tons  of  urea  are  excreted 
daily  by  the  human  race.  To  this  quantity  must  be  added  the  far 
greater  amount  excreted  by  other  animals,  for  all  animals,  large 

5 


50      NITROGEN.      DECOMPOSITION  OF  NITROGENOUS  COMPOUNDS. 

and  small,  secrete  it  or  an    allied  substance,  and  the  total  is  enor- 
mous.    What  becomes  of  it  all? 

Thus  the  nitrogen  of  the  nitrate  absorbed  by  the  plant  has 
reached  two  quite  different  conditions.  Part  of  it  is  still  in  the 
highly  complex  form  of  proteid,  either  in  the  dead  body  of  the 
animal  or  the  plant.  A  second  part  has  been  partly  broken  down 
in  its  passage  through  the  animal's  body,  and  has  reached  the  con- 
dition of  urea  or  some  allied  body.  But  in  neither  condition  is  it 
within  reach  of  another  generation  of  green  plants.  It  must  be 
still  further  broken  down  before  it  is  available  for  plants. 

ORGANIC  NITROGEN.    ITS  DECOMPOSITION. 

Decomposition  in  General. — This  means  the  breaking  to 
pieces  of  complex  compounds  so  as  to  form  simpler  ones.  The 
term  thus  defined  is  a  very  broad  one,  and  covers  a  long  series  of 
changes,  of  a  purely  chemical  nature.  But  more  commonly  the 
term  has  a  narrower  meaning,  and  refers  to  the  breaking  down  of 
organic  products  under  the  influence  of  microorganisms.  This  is 
one  of  the  most  important  functions  of  soil  bacteria.  The  destruc- 
tion of  nitrogenous  compounds,  urea  proteids,  gelatins,  or  other 
bodies,  is  brought  about  by  several  agencies,  but  the  chief  one  is 
undoubtedly  that  of  microorganisms.  A  small  amount  of  the  pro- 
teid appears  to  be  decomposed  in  plant  tissue  without  the  aid  of 
bacteria;  another  portion  is  broken  down  by  yeasts;  another  by 
molds  and  other  fungi.  But  decomposition  is  chiefly  due  to  a  class 
of  bacteria  called  the  decomposition  bacteria. 

But  even  as  thus  limited,  this  term  is  still  a  broad  one  including 
different  species  of  bacteria  and  various  types  of  decomposition. 
Two  types  are  generally  recognized,  under  the  names  of  decay 
and  putrefaction.  These  two  terms  are  frequently  not  very  clearly 
distinguished,  being  used  indiscriminately  to  refer  to  the  decomposi- 
tion of  organic  substances  under  the  influence  of  bacteria.  There 
is  a  distinction  between  them,  however,  which  may  be  properly 
drawn. 

Putrefaction. — This  is  the  name  given  to  a  partial  decomposi- 


ORGANIC   NITROGEN.      ITS    DECOMPOSITION.  5 1 

tion  that  is  far  from  complete.  It  is  generally  produced  by  bacteria 
growing  in  the  absence  of  oxygen,  and  hence  by  the  anaerobic  or 
facultative  anaerobic  bacteria.  These  break  down  the  proteids, 
but  do  not  carry  the  decomposition  to  its  final  stages,  the  final  pro- 
duct, thus  formed,  being  still  quite  complex.  Many  of  them  have 
unpleasant  odors  and  many  of  them  are  poisonous. 

Decay. — This  is  the  type  of  complete  decomposition  that  takes 
place  in  the  presence  of  oxygen.  It  is  produced  by  aerobic  bacteria, 
and  results  in  a  very  complete  disintegration  of  the  decomposing 
body.  The  end-products  are  much  simpler  than  in  the  case  of 
putrefaction,  and  the  gaseous  products  arising  have  little  or  no  odor. 
C02,N  and  H2O  are  among  these  final  products,  and  are  all  odorless. 
Putrefaction  and  decay  cannot  be  sharply  separated  from  each  other, 
the  former  being  in  many  cases  only  a  step  toward  the  latter.  The 
bad-smelling  or  poisonous  products  of  putrefaction  will,  if  exposed 
to  the  air,  undergo  further  disintegration  until  the  decay  is  complete. 
But,  though  not  sharply  distinct,  the  difference  above  noted  is  a 
convenient  method  of  designating  the  complete  decomposition, 
in  the  presence  of  air,  from  the  incomplete  decomposition  in  the 
absence  of  air. 

Of  the  many  species  of  bacteria  associated  with  putrefaction 
and  decay,  some  are  likely  to  be  found  under  one  set  of  conditions 
and  others  under  different  conditions.  Some  are  particularly 
common  in  decaying  vegetable  substances  and  others  in  decaying 
animal  tissues,  while  some  are  most  characteristic  in  fermenting 
urea.  No  attempt  need  be  made  here  to  classify  this  miscellaneous 
host  of  putrefactive  organisms.  They  include  cocci,  bacilli,  and  spiral 
forms  as  well  as  yeasts  and  higher  fungi  (Figs.  14,  15).  Some 
of  them  produce  their  fermentation  only  when  oxygen  is  present, 
while  others  do  so  in  the  absence  of  oxygen,  and  the  by-products 
produced  in  the  absence  of  oxygen  are  different  from  those  produced 
in  its  presence,  since  the  former  are  more  likely  to  be  of  a  poisonous 
nature.  These  decomposition  bacteria  occur  practically  everywhere 
in  nature — in  the  air,  in  all  bodies  of  water,  and  in  extreme  abund- 
ance in  the  soil.  They  are  so  widely  distributed  and  so  abundant 
that  they  are  sure  to  seize  hold  of  any  bit  of  nitrogenous  organic 


52       NITROGEN.       DECOMPOSITION  OF  NITROGENOUS  COMPOUNDS. 

matter,  which,  having  become  lifeless,  can  serve  them  as  food. 
Every  bit  of  excreted  urea,  even  that  secreted  by  the  smallest  insects, 
every  dead  animal  body,  every  bit  of  vegetable  matter  whether  it 
be  leaf,  branch,  or  fruit,  provided  it  contain  proper  moisture,  is  sure 
to  be  appropriated  as  food  by  some  of  these  ubiquitous  putrefactive 
bacteria.  The  material  is  used  as  food  by  the  microorganisms,  and, 
as  a  consequence,  they  multiply  rapidly  within  the  decaying  sub- 
stances, developing  vigorously  for  a  time.  After  they  have  used 
up  the  food,  their  growth  is  checked  and  some  of  them  remain 
ready  to  grow  again  when  more  organic  matter  comes  within  their 


Fig.   14.— Proteus  vul-  Fig.    15.— Com- 

garis,  a  common  bacter-  mon  decomposition 

ium  of  decomposition.  bacteria.    B.  fluor- 

escens  and  B.  sub- 
tilis. 


reach.     By  their  action,  then,  every  bit  of  organic  matter  which 
reaches  the  soil  is  seized  and  rapidly  decomposed. 

The  chemical  nature  of  these  destructive  changes  is  very 
complicated  and  highly  varied.  It  will  be  a  long  time  before  our 
chemists  understand  them,  for  they  involve  problems  in  physiolog- 
ical and  organic  chemistry  yet  unsolved.  We  know  that  many  new 
products  are  formed,  and  that  these  new  products  must  be  regarded 
as  belonging  to  at  least  two  types,  so  far  as  concerns  their  relation 
to  the  bacteria.  Some  of  them  must  be  regarded  as  secretions  or 
excretions  from  the  bacteria  and  hence  as  the  result  of  the  active 
metabolism  of  the  microorganisms.  These  are  probably  rather 
small  in  amount,  but  of  great  significance  in  some  connections, 
inasmuch  as  many  of  them  are  poisonous.  Others  must  be  looked 
upon  as  by-products  of  decomposition.  By  this  is  meant  that,  as 
the  bacteria  take  certain  atoms  from  the  complex  molecules  for 
their  own  use,  the  rest  of  the  molecule  can  no  longer  retain  its 


ORGANIC    NITROGEN.       ITS    DECOMPOSITION.  53 

earlier  form,  and  consequently  its  atoms  must  enter  into  new  relations 
to  form  new  bodies.  These  by-products  have  not  been  actually  in  the 
bacteria  and  are  not  the  direct  results  of  metabolism.  The  new 
products  formed  in  the  decomposing  mass  are  partly  gaseous. 
This  is  proved  by  the  odor  that  commonly  arises  from  putrefying 
bodies  which  are  indications  of  the  exhalation  of  volatile  products. 
A  chemical  study  has  shown,  in  many  cases,  the  actual  nature  of 
these  gaseous  products,  indicating  that  the  end-products  are 
chiefly  CO2,  H,  CH^,  NH3,  H^S,  and  N,  in  addition  to  others,  present 
in  much  smaller  amount,  producing  the  peculiar  and  characteristic 
odors.  Some  of  the  new  products  are  solids  and  may  be  either 
soluble  or  insoluble.  If  soluble  they  are  dissolved  in  the  course 
of  time  by  the  rain  which  falls  upon  the  decaying  mass  and  pass 
into  the  soil,  perhaps  to  be  drained  away  in  the  drainage-water. 
The  insoluble  bodies  are  also  incorporated  into  the  soil,  becoming 
eventually  mixed  with  the  solid  masses  of  the  earth. 

The  list  of  the  by-products  of  such  decompositions  is  a  long  one. 
A  few  of  these  are  as  follows :  Carbon  'dioxid,  hydrogen  sulphid, 
marsh  gas,  hydrogen,  nitrogen,  calcium  carbonate,  propionic  acid, 
valerianic  acid,  acetic  and  lactic  acids,  alcohol,  succinic  acid, 
phenol,  indol,  leucin,  tyrosin,  skatol,  etc. 

This  list  is  far  from  complete.  It  includes  only  a  few  of  the 
products  already  known,  and  beyond  question  there  are  numerous 
bodies,  formed  as  by-products  or  excretions,  which  still  remain 
to  be  discovered.  The  actual  products  which  appear  will  depend 
upon  three  factors:  (i)  The  substance  which  is  decaying;  (2) 
the  species  of  bacteria  which  produces  the  decay;  (3)  the  conditions 
under  which  the  decay  occurs. 

The  Ammoniacal  Fermentation. — One  phase  of  these  decom- 
position processes  must  be  especially  mentioned.  After  passing 
through  an  unknown  series  of  intermediate  stages,  the  nitrogen 
of  the  decaying  mass  assumes,  in  large  part,  the  condition  of 
ammonia.  One  of  the  first  and  easiest  substances  to  undergo 
this  ammoniacal  fermentation  is  urea.  Urine  is  always  filled 
with  bacteria,  even  while  in  the  ducts  from  the  bladder,  and  among 
them  are  several  species  that  cause  it  to  break  down  to  form  ammonia 


54      NITROGEN.       DECOMPOSITION  OF  NITROGENOUS  COMPOUNDS. 

(Fig.  1 6).  Most  of  these  bacteria  produce  this  action  through  an 
enzyme  that  they  secrete,  named  urase.  Under  proper  conditions, 
as  much  as  97  per  cent,  of  the  nitrogen  in  the  urea  is  converted  into 
ammonia  in  a  space  of  four  days.  The  ammonia  is  a  volatile 
product  and  has,  consequently,  a  tendency  to  pass  off  into  the  air, 
as  may  readily  be  recognized  from  the  odor  of  ammonia  that  is 
frequently  perceived  around  a  manure  pile.  This  represents  a  per- 
manent loss  of  nitrogen,  and  should  be  avoided  as  much  as  possible. 


Fig.  16. — Various  bacteria  causing  the  ammoniacal  fermentation  of  urea  (Beijerinck). 

The  loss  is  greater  when  the  liquid  is  concentrated,  and  consequently 
less  if  the  urine  can  be  poured  upon  the  soil  at  once,  than  if  stored 
in  vats  or  even  mixed  with  solid  manure. 

Although  urea  shows  the  ammoniacal  fermentation  most  readily, 
other  nitrogenous  bodies,  like  proteids,  etc.,  also  may  give  rise  to 
ammonia  (Fig.  17),  which  is,  indeed,  one  of  the  common  end-products 
of  proteid  decomposition.  The  chemical  changes  that  occur  in 
proteid  decomposition  are  complex  and  not  wholly  understood. 
The  first  step  seems  to  be  quite  like  that  taken  when  they  are 
digested  in  the  digestive  tract  of  animals,  for,  under  the  action  of  the 


ORGANIC    NITROGEN.      ITS    DECOMPOSITION.  55 

peptonizing  bacteria,   they  are  converted   into  peptone-Wke  bodies 

which   are   simpler   than   ordinary   proteids.     These   are   further 

reduced  into  amido  acids,  and  the  latter  finally  converted  into 

ammonia,  which  is  quite  likely  to  unite  at  once  with  carbonic 

dioxid,  that  is  also  being  liberated,  to  form  carbonate  of  ammonia 

((NH4)2C03).     The  condition  in  which  the  nitrogen  actually  exists 

in  the  humus  has  been  a  matter  of  considerable  dispute.     Some 

have  thought  that  it  remains  in  the  form  of  proteid  in  large  part, 

others  have  concluded  that  the  humus  nitrogen 

is  chiefly  in  the  form  of  amido  acids.     But  it 

is  evident  that  there  is  long  series  of  stages 

between   the  proteid  and  the  final  ammonia 

compound,  and  that  the  nitrogen  in  any  lot 

of  soil  may  be  in  any  one  of  these  stages.     In 

whatever  form  it  exists  in  the  humus,  a  certain       ^  -o    .   • 

'  Fig  .  1 7 . — Bactena  pro- 

portion of  it  is  being  constantly  reduced  to  the     ducing  ammoniacal  fer- 

form  of  ammonia.  This  portion  alone  is  lead-  ^^^^^^b'b-  stutzerC^' 
ing  toward  a  condition  where  it  can  again  be 
utilized  by  plants.  The  rest,  whether  in  the  form  of  proteid  or 
otherwise,  is,  for  the  present,  locked  up  out  of  the  reach  of  plant 
life.  The  humus  may  thus  contain  a  large  amount  of  nitrogen  and 
still  have  little  of  it  available;  i.e.,  within  the  reach  of  plants. 

Self-purification  of  the  Soil. — The  universal  occurrence  of 
such  a  decomposition  of  organic  bodies  is  no  new  discovery.  It 
has  long  been  known  and  its  extreme  significance  is  now  recognized, 
since  it  is  the  first  step  necessary  to  bring  the  nitrogen  locked  up 
in  the  proteid  back  again  within  reach  of  plants.  But  its  value 
in  producing  what  has  been  called  the  self-purification  of  the  soil, 
has  been  only  recently  appreciated.  As  we  have  seen,  the  final 
end-products  are  largely  gaseous  (NH3,C02,N,  etc.),  and  these 
will  tend  to  pass  off  from  the  soil  into  the  air.  A  little  thought  will 
show  us  that  without  the  existence  of  some  such  process  the  soil 
would  rapidly  become  unfit  for  the  support  of  life  simply  by  becom- 
ing clogged  up  with  the  remains  of  past  animals  and  plants.  If 
all  the  bodies  of  animals  remained  on  the  soil  after  death  and  if 
the  roots  and  stems  of  plants  were  not  disposed  of  by  some  such 


56      NITROGEN.       DECOMPOSITION  OF  NITROGENOUS  COMPOUNDS. 

process,  it  is  evident  enough,  from  simple  mechanical  reasons,  that 
vegetation  would  soon  cease,  since  there  would  be  no  room  left  in 
the  soil  for  new  plants.  When  we  realize,  in  addition,  that  the 
very  processes  which  purify  the  soil  of  these  cumbersome  bodies  are 
bringing  them  toward  a  condition  for  further  use,  we  can  appre- 
ciate the  extreme  significance  of  these  decomposition  bacteria  in 
agriculture. 

All  the  types  of  decomposition  which  we  have  mentioned  take 
place  in  the  humus  of  the  soil;  sugars,  starches,  cellulose,  woody 
tissues,  proteids,  and  all  other  kinds  of  organic  bodies  are  attacked 
by  microorganisms  and  eventually  thoroughly  decomposed.  Since 
this  decomposition  is  the  first  step  in  the  conversion  of  the  products 
of  one  generation  of  living  things  toward  the  condition  in  which  they 
can  again  be  used,  the  conditions  of  the  soil  should  be  such  as  to 
favor  such  decomposition.  The  thorough  decay  is  possible  only  in 
the  presence  of  oxygen,  and  hence  a  cultivation  of  the  soil  facilitates 
decomposition.  Hard  packed  soils  are  inferior  to  looser  soils 
for  this  reason.  The  presence  of  large  amounts  of  carbohydrates, 
sugars,  starches,  straw,  etc.,  is  apt  to  give  rise  to  acids,  and  soils 
containing  them  may  become  sour.  In  such  soil  the  nitrogenous 
decomposition  is  checked,  since  decomposition  bacteria  cannot 
stand  much  acid.  It  is  evident,  therefore,  that  in  sour  soils  the 
addition  of  lime  to  neutralize  the  acid  will  make  it  possible  for 
the  bacteria  to  carry  on  an  active  decomposition  that  will  soon  place 
its  food  materials  once  more  within  the  reach  of  plant  life.  The 
more  vigorous  the  decomposition  changes,  roughly  speaking, 
the  higher  the  fertility  of  the  soil.  Black  marsh  soil  shows  the  highest 
amount  of  decomposition;  clay  shows  less,  as  a  rule,  and  sandy 
soil  the  least.  The  number  of  bacteria  in  any  soil  is  directly 
proportional  to  the  activity  of  the  decomposition  changes  going  on 
within  it. 


CHAPTER  V. 
NITRIFICATION  AND  DENITRIFICATION. 

NITRIFICATION. 

The  pulling  of  the  organic  nitrogen  compounds  to  pieces  does 
not  in  itself  bring  the  nitrogen  into  the  best  available  condition  for 
plants.  It  is  in  the  form  of  nitrates  that  plants  most  readily  absorb 
nitrogen,  and  at  the  end  of  the  decompositions  noticed  ammonia 
compounds  are  formed,  but  no  nitrates.  Plants  may  be  able  to 
absorb  nitrogen  in  the  form  of  ammonia  salts,  but  this  occurs  only 
to  a  slight  extent,  and  by  far  the  largest  amount  is  assimilated  in 
the  form  of  nitrates.  Consequently,  if  these  decomposition  pro- 
ducts are  to  be  utilized  by  plants,  they  need  to  be  changed  from 
ammonia  salts  into  nitrates.  This  process  has  been  called  nitri- 
fication. 

Nitrification  is  a  process  of  oxidation.  In  the  oxidation  of  am- 
monia compounds  to  form  nitrates  there  are  two  separate  stages. 
The  first  is  one  by  which  the  ammonia  is  oxidized  into  a  nitrite. 
A  nitrite  is  a  salt  of  nitrous  acid  (HNO2),  and  it  contains  less  oxygen 
than  a  nitrate.  Nitrites  are  not  plant  foods,  for,  as  far  as  known, 
ordinary  plants  never  absorb  nitrogen  in  this  form.  The  second 
change  is  the  addition  of  another  atom  of  nitrogen  to  the  nitrite, 
giving  a  nitrate  or  salt  of  nitric  acid  (HNO3),  the  form  in  which  the 
nitrogen  is  most  completely  available  for  plants. 

Nitrates  are  really  of  very  great  significance  in  nature.  They 
are  readily  soluble  in  water,  so  that  they  are  easily  taken  up  by  the 
soil  and  absorbed  by  the  roots;  thus  nitrates  feed  the  whole  world  of 
green  plants.  In  addition  to  this,  nitrates  form  the  basis  of  most 
explosives.  Gunpowder  has  saltpeter  as  its  basis,  and  saltpeter  is 
nitrate  of  potash.  Nitroglycerin,  too,  is  made  from  nitric  acid,  and 
practically  all  the  other  commonly  used  explosives  are  produced 

57 


58  NITRIFICATION    AND    DEN ITRIFI CATION. 

from  nitrates.  Nitrate  formation  is,  then,  a  matter  of  the  greatest 
significance.  While  there  are,  perhaps,  some  other  methods  by 
which  nitric  acid  can  be  formed,  beyond  doubt  the  nitrate  store  in 
the  soil  has  been  formed  chiefly  through  the  process  of  nitrification. 

Nitrification  in  the  Soil. — It  is  very  easy  to  demonstrate  that 
such  nitrification  actually  takes  place  in  ordinary  soil.  If  we  place 
a  quantity  of  soil  in  a  proper  vessel  and  subject  it  at  intervals  to  a 
chemical  analysis,  it  will  be  found  that  there  is  an  increase  in  the 
amount  of  nitrates  present,  after  it  has  remained  undisturbed  for  a 
few  weeks.  This  fact  has  been  known  for  nearly  a  century.  The 
next  step  in  the  discoveries  was  made  in  1877,  when  it  was  demon- 
strated that  this  nitrification  is  associated  with  the  presence  of  living 
matter  in  the  soil.  This  can  be  proved  by  placing  two  lots  of  the 
same  soil  under  such  conditions  that  in  the  one  phenomena  of  life 
may  go  on,  while  in  the  other  they  are  stopped.  If,  for  example,  one 
lot  of  soil  is  sterilized  by  heating  it  sufficiently  to  destroy  the  living 
germs  present,  and  then  this  soil  is  compared  with  another  lot 
treated  in  all  respects  the  same,  except  that  it  is  not  sterilized,  the 
latter  will  be  found  to  increase  its  nitrates,  while  the  former  will  show 
no  such  increase.  The  same  results  are  obtained  if  the  soil  is  mixed 
with  antiseptics  which  prevent  bacteria  growth.  In  short,  any- 
thing which  prevent-s  the  occurrence  of  life  phenomena  in  the  soil, 
prevents  the  nitrification. 

Isolation  of  the  Nitrifying  Organisms. — Such  experiments 
repeated  many  times  and  verified  by  numerous  observers  demon- 
strated that  nitrification  is  the  result  of  a  living  process.  Inasmuch 
as  such  soil  contains  no  plants  large  enough  to  be  seen,  it  follows  that 
the  living  agent  of  nitrification  must  be  some  form  of  microorganism. 
It  proved,  however,  to  be  a  very  dijficult  matter  to  find  the  organisms 
concerned  in  the  process.  The  number  of  bacteria  in  the  soil  is  large 
and  many  different  species  are  there  found.  But  although  many 
of  these  bacteria  were  isolated  and  carefully  tested,  for  a  long  time 
none  proved  to  have  any  power  of  nitrification.  Most  of  them,  indeed, 
produced  the  reverse  effect,  that  of  deoxidizing  nitrates,  but  none 
of  them  raised  the  nitrites  into  a  state  of  nitric  acid.  None  of  them 
could  oxidize  ammonia  so  as  to  form  nitric  or  even  nitrous  acid.     If 


NITRIFICATION. 


59 


a  small  quantity  of  soil  is  added  to  a  solution  of  nitrite  the  nitrite  soon 
becomes  converted  into  nitrate,  under  the  influence  of  the  fermenta- 
tion started  by  the  presence  of  the  soil.  This  shows  that  the  soil 
must  contain  the  nitrifying  organisms.  But  the  bacteria  which 
are  isolated  from  such  soil  by  ordinary  methods  showed  no  power 
of  nitrification.  Evidently  the  nitrifying  bacteria  cannot  be  found 
by  the  ordinary  bacteriological  methods. 

The  cause  of  the  trouble  as  well  as  the  secret  of  successful  study 
was  soon  learned.  In  bacteriological  studies  the  common  method  of 
isolating  bacteria  is  to  get  them  to  grow  in  culture  media  made  by 
the  bacteriologist.  The  media  commonly  used  contain  a  certain 
amount  of  organic  compounds  which  serve  as  food  for  the  bac- 
teria. But  experiment  soon  showed  that  the  presence  of  the  smallest 
amount  of  organic  matter  is  directly  injurious  to  the  nitrifying 
bacteria,  so  that  they  will  not  grow  at  all  in  ordinary  culture  media. 
It  was  necessary  to  devise  some  culture  media  that  contained  no 
organic  matter,  and  as  soon  as  this  was  done  it  was  possible  to 
isolate  from  the  soil  bacteria  having  the  power,  under  proper  con- 
ditions, of  oxidizing  ammonium  and  nitrite  compounds  into  ni- 
trates. For  a  while  the  results  of  experiments  were  in  some  con- 
fusion, since  in  some  cases  nitrates  appeared  to  be  formed,  while  in 
others  they  did  not.  It  became  evident  that  nitrification  was  not  a 
simple  phenomenon,  and  further  study  showed  that  the  nitrification, 
as  occurring  in  ordinary  soil,  is  a  two-fold  process.  The  first  step  in 
the  process  oxidizes  the  ammonia  into  nitrites.  In  most  of  the  ex- 
periments the  nitrogen  was  put  into  the  culture  fluids  in  the  form  of 
sulphate  or  carbonate  of  ammonia  and  this  was  readily  oxidized  into 
nitrite.  The  second  step  was  the  oxidation  of  the  nitrites  into 
nitrates.  The  two  steps  are  not  only  independent,  but  they  are 
brought  about  by  two  different  species  of  bacteria.  One  organism 
has  the  power  of  producing  nitrite  out  of  ammonia,  but  can  carry 
the  oxidation  no  farther,  failing  to  produce  nitrates.  The  second 
species  can  act  upon  the  nitrites,  carrying  their  oxidation  up  to  the 
form  of  nitrates,  but  it  has  no  power  to  act  upon  ammonia.  The 
two  together  can  produce  the  complete  nitrification  of  both  am- 
monium and  nitrite  compounds. 


60  NITRIFICATION    AND    DENITRIFICATION. 

The  Nitrifying  Bacteria.— It  thus  appears  that  there  are  two 
types  of  nitrifying  bacteria.  The  first  converts  ammonium  com- 
pounds into  nitrites,  and,  hence,  are  called  the  nitrite  bacteria 
(Fig.  18).  They  have  been  found  in  soils  of  very  widely  separate 
localities,  and  probably  live  in  all  soils.  Two  slightly  different 
varieties  have  been  recognized,  both  spherical  bacteria,  and  named 
Nitrosomonas  and  Nitrosococcus,  names  that  will  probably  soon  go 
out  of  use.  They  appear  to  be  able  to  form  nitrites  from  almost  any 
kind  of  ammonium  salt,  and,  since  they  are  quite  universally  dis- 
tributed in  all  decaying  organic  matter, 

*4C»  jl  *^*  #1  n     ^^  ^^^^  ^^  ^^  ^^^  humus,  they  will  evi- 

dently seize  the  ammonium  compounds 
produced  by  ammoniacal  decomposition, 
•  -^ /  '^  and  convert  them  into  nitrites  (Fig.  18,  A). 

•*•         ^A      c  They  are  incapable,  however,  of  forming 

*'<>R  nitrates   from   any   nitrogen    compound 

^  S{  tJ*  except  ammonium  salts,  and  hence  the 

Fig.  iS—Nitrifying  bacteria,     proteid  compounds  of  decaying  bodies 

^  IS  a  nitrous  and  B  and  C  •     .^     i     .,,     ,  ^  ^ 

nitric  bacteria.  cannot  be  nitrilied  till  they  are  reduced 

to  the  form  of  ammonia.  The  second 
type  of  nitrifying  bacteria  is  called  the  nitrate  bacteria,  since  they 
oxidize  the  nitrites  into  nitrates.  Only  a  single  type  of  this  class 
has  been  found,  and  it  was  named  Nitrobacter  (Fig.  18,  B  and  C). 
It  is  smaller  than  most  nitrite  organisms  and  of  a  slightly  elongated 
shape.  It  is  also  widely  distributed,  probably  in  all  soils,  and  is 
able  to  convert  any  kind  of  nitrite  into  nitrate.  It  cannot,  how- 
ever, act  upon  any  nitrogen  compounds  except  nitrites,  and  hence 
its  action  must  be  preceded  by  that  of  the  nitrite  bacteria. 

In  ordinary  soil  these  two  kinds  of  nitrifiers  act  together  and 
simultaneously.  So  closely  connected  is  their  action  that  it  is 
difficult  to  find  any  traces  of  nitiites  in  the  soil,  since  they  are 
converted  into  nitrates  as  rapidly  as  they  are  formed.  The  whole 
nitrification  may  be  very  rapid.  If  ammonium  salts  are  added  to 
soil,  they  cannot  commonly  be  found  in  the  drainage- water  from  the 
soil,  since  the  nitrification  progresses  so  rapidly  that  they  become 
completely   converted    into   nitrates   before   draining   away.     But 


CONDITIONS    OF   LIFE    OF   NITRIFYING    ORGANISMS.  6l 

though  occurring  simultaneously,  the  two  steps  in  the  nitrification 
appear  to  be  distinct  and  produced  by  distinct  organisms.  It 
has  been  claimed  recently  that  there  are  other  classes  of  soil  organisms 
that  take  these  two  steps  in  one,  converting  ammonia  and  even 
organic  matter  directly  into  nitrates.  If  this  be  true,  they  represent 
distinct  classes  of  nitrifiers,  but  the  observations  have  not  yet  been 
sufi&ciently  verified. 

CONDITIONS  OF  LIFE  OF  NITRIFYING 
ORGANISMS. 

The  importance  of  the  phenomenon  of  nitrification  makes  it 
very  desirable  to  understand  thoroughly  the  conditions  under  which 
it  may  best  occur,  and,  consequently,  the  means  for  stimulating 
or  hindering  it.  The  conditions  regulating  the  life  of  these  nitrifiers 
are,  in  some  respects,  peculiar. 

Organic  Food. — In  respect  to  food  these  nitrifiers  are  among 
the  most  remarkable  of  all  organisms.  Not  only  do  they  need  no 
organic  food,  but  the  presence  of  organic  matter  in  the  solutions,  even 
in  small  quantity,  is  directly  injurious.  The  bacteria  will  grow 
readily  in  mineral  solutions,  but  if  a  small  quantity  of  organic 
matter  is  added,  the  growth  stops.  In  ordinary  laboratory  solutions 
a  very  small  amount  of  organic  matter  acts  like  an  antiseptic.  In 
the  soil,  however,  the  nitrifiers  behave  differently,  and  are  not 
checked  in  their  growth  by  such  small  quantities  of  organic  matter 
as  serve  to  check  them  in  laboratory  solutions.  This  injurious 
action  of  organic  matter  is  a  curious  phenomenon.  The  more 
highly  organized  the  compound,  the  more  decided  its  checking 
action,  and  thus,  the  more  valuable  the  material  for  ordinary  kinds 
of  bacteria,  the  greater  its  injury  upon  the  nitric  bacteria.  These 
bacteria  thus  grow  under  conditions  detrimental  to  other  bacteria, 
but  will  not  grow  under  the  conditions  which  other  species  find 
most  favorable.  A  more  sharp  contrast  can  hardly  be  conceived. 
Not  only  bacteria,  but  all  other  colorless  plants  are  obliged  to  depend 
upon  organic  food  as  a  source  of  energy,  in  this  respect  resembling 
animals.     But  here  is  a  group  of  organisms  that  not  only  does  not 


62  NITRIFICATION    AND    DENITRIFICATION. 

need,  but  cannot  grow  in  the  presence  of,  organic  matter.  They  do 
not,  therefore,  need  any  other  living  organisms  to  interpose  between 
them  and  the  mineral  world,  but  may  develop  under  conditions  in 
which  they  are  supplied  with  mineral  substances  alone.  It  is 
more  surprising  perhaps  to  find  that  they  do  not  need  light,  but  can 
utilize  the  mineral  substances  while  growing  in  perfect  darkness. 
This  fact  was  at  first  conceived  as  quite  contrary  to  our  general 
ideas  of  the  relation  of  life  to  physical  energy.  We  have  supposed 
that  the  only  source  of  energy  for  living  things  is  sunlight,  and  that 
this  energy  is  stored  up  by  green  plants  in  the  form  of  chemical 
compounds  of  high  complexity.  The  animals  and  colorless  plants 
use  these  stores  as  food,  breaking  them  up  and  using. the  energy 
liberated  for  their  own  use.  But  here  we  have  organisms  which  do 
not  require  organic  material  as  a  source  of  energy  and  are  not  able 
to  utilize  sunlight  itself  directly.  Evidently  they  must  obtain  their 
energy  from  some  other  source  than  that  which  is  commonly  utilized 
by  animals  and  plants.  That  they  have  a  source  of  energy  at  com- 
mand is  evident  from  the  fact  that  they  can  assimilate  CO2  and 
build  it  into  their  own  tissues,  a  process  that  requires  energy.  The 
present  belief  is  that  they  obtain  their  energy  from  the  oxidation 
of  the  ammonia  compounds,  a  process  that  apparently  can  furnish 
them  with  all  they  need.  But  whatever  its  source,  these  nitrifiers 
are  able  to  live  under  conditions  in  which  other  organisms  cannot 
exist. 

Since  the  nitrifiers  are  injured  by  organic  matter,  it  follows  that 
nitrification  cannot  be  expected  in  highly  concentrated  decompos- 
ing masses.  Raw  sewage  contains  so  much  high  organic  matter 
that  nitrification  does  not  take  place  in  it,  and  if  it  is  applied  directly 
to  the  soil,  in  considerable  quantity,  it  will  effectually  prevent  the 
nitrification  necessary  to  render  the  nitrogen  available  to  plants. 
In  the  manure  heap,  too,  nitrification  cannot  be  expected  so  long  as 
the  quantity  of  organic  matter  is  high. 

As  the  manure  rots,  however,  the  organic  nitrogens  are  reduced, 
until  finally  nitrification  can  begin.  We  have  seen  that  decomposi- 
tion gives  rise  to  ammonia,  usually  combining  with  carbonic  acid 
to  form  ammonium  carbonate.     This  compound  is  also  injurious 


CONDITIONS    OF   LIFE   OF   NITRIFYING    ORGANISMS.  63 

to  the  nitrifiers  and,  if  it'becomes  too  abundant,  will  stop  nitrification 
until,  either  by  vaporization  or  by  denitrification,  or  otherwise, 
it  is  reduced  to  an  amount  not  deleterious  to  nitrification,  when 
the  process  begins.  This  checking  of  the  action  of  nitrifiers  by 
organic  matter  or  ammonia,  certainly  occurs  in  laboratory  solutions 
and  concentrated  compost  heaps,  but  it  does  not  appear  to  be  of 
much  significance  in  ordinary  soil.  Nitrification  takes  place  more 
vigorously  in  soil  than  in  solutions,  and  proper  testing  has  shown 
that  the  organic  matter  in  ordinary  soil  does  not  prevent  a  vigorous 
nitrification. 

Moisture. — The  nitrifiers  require  a  moderate  amount  of  moisture. 
Too  dry  a  soil  will  not  allow  of  their  growth.  But,  on  the  other  hand, 
too  much  moisture  is  equally  detrimental.  It  has  just  been  stated 
that  they  do  not  grow  so  readily  in  laboratory  solutions  as  in  soil. 
It  is  also  a  fact  that  in  very  wet  soil,  ''water-logged,"  nitrification 
is  greatly  reduced  or  lacking. 

Reaction. — The  nitrifiers  cannot  develop  in  an  acid  medium 
and  are  usually  absent  from  acid  soils.  Soils  may  become  acid 
from  various  causes,  one  of  the  chief  of  which  is  the  production  of 
certain  organic  acids  {lactic,  succinic,  acetic,  butyric,  etc.),  from  the 
bacterial  decomposition  of  carbohydrate  material.  Large  amounts 
of  sugar  might  give  rise  to  an  acid  condition,  but  the  more  common 
cause  is  the  decomposition  of  cellulose  and  woody  substances.  In 
forest  land  the  decay  of  leaves  and  branches,  as  well  as  other  vege- 
table structures  containing  cellulose  material,  usually  fills  the  soil 
with  these  acids  and,  as  a  result,  nitrification  has  practically  ceased 
in  forest  soils.  The  same  is  true  in  some  open  pastures  and  other 
soils  where  such  decay  is  extensive.  The  value  of  liming  such  soils 
is  evident.  Lime  neutralizes  the  acids  and  restores  the  alkaline 
condition  necessary  for  the  nitrifiers,  so  that  they  may  resume  the 
activity  stopped  by  the  acid.  Too  much  lime,  however,  defeats  its 
end  by  making  the  soil  too  alkaline. 

Humus. — That  nitrification  may  take  place,  it  is  of  course 
necessary  that  there  be  plenty  of  nitrogenous  material  to  be  nitrified. 
This  must  be  in  the  form  of  an  ammonium  salt  which,  as  we  have 
seen,  is  the  condition  reached  by  the  organic  nitrogen  at  the  end  of 


64  NITRIFICATION    AND    DENITRIFICATION. 

its  decomposition.  The  ordinary  humus  will  therefore  furnish 
plenty,  but  soil  deficient  in  humus  will  show  but  little  nitrification. 

Temperature. — Nitrification  occurs  in  the  soil  under  a  very 
wide  range  of  temperature.  It  goes  on  at  temperatures  fully  as  low 
as  37°  F.;  it  is  most  vigorous  at  about  99°,  becomes  manifestly 
checked  at  110°,  and  almost  ceases  at  122°.  From  these  facts  it  will 
be  seen  that  it  may  continue  in  the  fall  until  the  appearance  of 
frosts  and,  in  many  localities  where  the  winter  is  not  too  cold,  will  go 
on  all  winter  long.  For  this  reason  a  cultivation  of  soil  in  the  fall 
is  undesirable,  since  cultivation,  by  mixing  air  with  soil,  hastens 
nitrification,  and  during  the  winter  or  late  fall  there  is  no  growing 
crop  to  utilize  the  nitrates  as  they  are  formed.  These,  therefore, 
drain  away  from  the  soil  during  the  spring  and  winter,  leaving  it 
poorer  in  the  spring  than  if  the  cultivation  had  not  taken  place. 
Nitrification  is  the  most  vigorous  in  the  summer  months,  during 
which  season  the  growing  crops  are  in  best  condition  for  absorbing  it. 
This  is  one  of  the  reasons  why  a  wheat  crop  is  so  exhausting  to  the 
soil.  It  grows  during  the  fall  and  spring,  but  the  ground  lies  idle 
in  the  summer  and  hence  during  the  season  of  greatest  formation 
of  nitrate,  there  is  no  crop  growing  to  prevent  the  loss  by  drainage. 

Air. — Nitrification  is  a  process  of  oxidation  and  therefore  re- 
quires oxygen.  The  more  thoroughly  the  air  is  mixed  with  the  soil 
the  more  vigorous  will  be  the  nitrification.  This  process,  therefore, 
is  more  pronounced  in  sandy  loams  or  mixtures  of  clay  and  sand 
than  it  is  in  heavy  clay  soils.  In  heavy  soils,  where  the  earth 
particles  are  very  fine,  the  soils  are  too  poorly  aerated  to  enable  the 
nitrifiers  to  get  a  sufficiency  of  oxygen.  From  this  we  learn  the 
very  practical  lesson  that  cultivation  of  the  soil  stimulates  nitrifica- 
tion. Experience  and  theory  both  tell  that  the  loosening  up  of  soil 
during  the  growth  of  plants  greatly  stimulates  plant  growth,  and  the 
primary  reason  is  evidently  because  this  furnishes  the  necessary 
oxygen  for  a  vigorous  nitrification,  thus  furnishing  the  crops  with  a 
larger  supply  of  the  easily  assimilated  nitrates.  In  this  fact,  too,  we 
find  an  explanation  of  the  fact  that  only  the  upper  layers  of  the  soil 
are  fertile  since  nitrification  will  go  on  only  in  the  layers  where 
oxygen  readily  penetrates.     About  65  per  cent,  of  the  total  nitrifica- 


EXTENT    OF   NITRIFICATION.  65 

tion  occurs  in  the  upper  twelve  inches  of  soil,  30  per  cent,  more  in  the 
layers  from  twelve  to  thirty-six  inches  lower,  and  little  or  none  be- 
low this.     Surface  soils  alone  are  thus  highly  fertile. 

EXTENT  OF  NITRIFICATION. 

The  production  of  nitrates  in  ordinary  soil  is  very  vigorous. 
While  in  some  soils  it  does  not  occur  at  all,  from  a  lack  of  some  of  the 
conditions  already  mentioned,  in  other  soils  nitrates  are  rapidly 
formed.  In  fact,  a  much  larger  amount  of  nitrates  is  produced  in  a 
cultivated  soil,  ordinarily,  than  is  used  by  the  crops.  In  some  care- 
ful tests  it  has  been  shown  that  twice  as  much  nitrate  is  formed  as  is 
used  by  the  crop,  the  rest  being  lost  to  the  soil  by  drainage.  This  is 
particularly  true  when  wheat  is  grown,  wheat  being  an  especially 
exhausting  crop.  To  furnish  this  amount  of  nitrate  a  proper  amount 
of  organic  nitrogen  must  be  added  in  the  form  of  manure  or  other- 
wise. With  plenty  of  such  material  as  a  source,  nitrification  is  very 
vigorous  during  all  seasons,  except  when  the  soil  is  actually  frozen. 

The  Unlocking  of  Soil  Nitrogen. — It  happens  not  infre- 
quently that  soil  may  contain  large  amounts  of  nitrogen  and  yet 
fail  to  produce  good  crops,  the  plants  seeming  to  be  insufficiently 
supplied  with  nitrogen  in  spite  of  its  abundance.  These  barren 
soils  will  not  yield  good  crops  unless  supplied  with  a  considerable 
amount  of  nitrogen  as  a  fertilizer.  Upon  an  open  hillside  or  a 
meadow  we  may  find  the  land  very  poor  for  supporting  vegetation, 
and  yet  its  soil,  when  analyzed,  may  yield  a  considerable  quantity 
of  nitrogen.  In  such  a  soil  the  nitrogen  is  simply  locked  up  in  the 
humus  in  a  form  useless  to  plants.  At  the  end  of  decomposition, 
a  large  part  of  the  nitrogen  may  be  held  in  a  form  not  available  for 
ordinary  vegetation,  so  that  plants  growing  in  such  soil  will  be 
nitrogen-starved,  although  growing  in  the  midst  of  plenty  of  nitrogen 
compounds.  Such  soils  might  become  highly  fertile  if  some  agency 
for  unlocking  these  nitrogenous  compounds  could  free  the  nitrogen 
from  its  stable  relations,  thus  producing  compounds  of  a  nature  to  be 
assimilated  by  plants.  A  nitrification  is  evidently  what  is  needed  to 
make  these  soils  productive.  If  a  comparatively  small  amount  of 
6 


66  NITRIFICATION    AND    DENITRIFI CATION. 

manure  is  added  to  such  soils  the  results  are  sometimes  surprising 
in  causing  an  increased  fertility  far  beyond  that  which  might  be 
expected  from  the  small  amount  of  manure  itself. 

For  example,  one  frequently  sees  that  an  open  pasture  or  meadow 
supports  a  somewhat  limited  crop  of  grass,  although  nitrogen  com- 
pounds may  be  abundant  enough  in  the  soil.  If  cows  are  pastured 
there  it  is  common  to  find  plots  of  brilliant  green,  vigorously  growing 
vegetation,  surrounding  the  droppings  of  the  cow  excrement.  Now 
this  may  be  due  in  part  to  the  food  contained  in  the  excrement 
which  is  utilized  by  the  plant,  but  it  is  not  wholly  thus  explained. 
The  effect  lasts  for  a  long  time,  and  months  afterward  the  oasis  of 
green  may  be  seen  in  the  pasture,  gradually  increasing  in  size  until 
it  reaches  far  beyond  what  must  have  been  the  limits  of  the  direct 
effect  of  the  plant  food  in  the  excrement.  The  explanation  seems 
to  be  that  by  this  excrement  the  nitrifying  bacteria  are  stimulated, 
and  these  in  a  short  time  begin  the  work  of  converting  the  soil 
nitrogens  into  nitrates.  Their  influence  continues  to  extend  through 
the  soil  as  they  multiply  and  act  upon  a  wider  and  wider  circle,  so 
that  an  increased  vegetation  may  continue  for  a  long  time  under 
the  influence  of  these  nitrifying  bacteria  which  are  constantly  con- 
verting the  soil  nitrogens  into  nitrates.  That  this  is  the  whole  ex- 
planation in  these  cases  is  by  no  means  sure,  but  it  is  certain  that  the 
nitrifiers  do  unlock  much  nitrogen  previously  not  in  an  available 
condition. 

DENITRIFICATION. 

There  is  another  group  of  microorganisms  in  soil  and  other 
decaying  masses  acting  in  exactly  the  reverse  direction  from  the 
nitrifiers.  Whereas  nitrification  oxidizes  ammonia  compounds  and 
nitrites,  to  form  nitrates,  denitrification  takes  the  oxygen  out  of 
nitrates,  reducing  them  to  nitrites  and  ammonia,  and  may  even 
reduce  these  to  free  nitrogen.  Nitrification  prepares  plant  foods, 
but  denitrification  destroys  them.  The  one  process  is  useful,  the 
the  other  detrimental,  to  soils 

Three    different    types    oi    reduction   of   nitrogen   compounds 


DENITRinCATION.  67 

are  comprised  under  this  head:  i.  The  reduction  of  nitrates  into 
nitrites.  2.  The  reduction  of  nitrates  to  give  ofiF  free  nitrogen.  3. 
The  reduction  of  nitrites  into  free  nitrogen.  The  term  denitrifica- 
tion  is  sometimes  used  to  cover  all  of  these  types  of  reduction,  and 
sometimes  more  particularly  to  refer  only  to  the  reduction  of  the 
nitrates  and  nitrites,  so  as  to  liberate  free  nitrogen.  In  its  strict 
use  it  should  be  confined  to  the  latter  process. 

It  is  evident  that  these  different  types  of  reduction  will  have 
different  effects  upon  soil  fertility.  Those  portions  of  the  nitrogen 
that  are  reduced  to  nitrates  or  ammonia,  under  proper  conditions 
may  be  built  up  again  into  nitrates  by  the  nitrifying  bacteria.  But 
those  that  are  reduced  to  a  condition  of  free  nitrogen  pass  off 
into  the  air  and  out  of  the  reach  of  plant  life.  This  nitrogen, 
therefore,  represents  an  actual  loss  to  the  soil.  Denitrification 
is  a  process  very  different  from  the  general  type  of  decomposition 
which  we  have  described.  Decomposition  begins  with  proteids  and 
reduces  them  to  ammonia  compounds.  Denitrification  begins  with 
nitrates  and  nitrites,  and  liberates  free  nitrogen. 

The  Denitrifying  Bacteria. — Denitrification  is  the  result 
of  the  action  of  a  class  of  bacteria  known  as  the  denitrifiers.  Very 
many  bacteria  have  the  power  of  extracting  the  oxygen  from  nitrates, 
reducing  them  to  nitrites,  but  the  list  of  those  that  can  liberate  free 
nitrogen  is  shorter.  Some  of  them  act  in  aerobic  conditions,  and 
others  in  anaerobic  conditions.  The  names  B.  denitrificanss. 
I  and  II  have  been  given  to  two  of  them,  but  others  have  been  found 
with  similar  properties.  They  are  very  widely  distributed;  they 
are  found  not  only  in  soil  and  water,  but  in  the  air  and  all  organic 
decomposing  refuse.  They  are  very  abundant  in  the  manure  heap, 
especially  if  it  contains  much  hay  and  straw,  and  they  are  likely 
to  cause  a  considerable  loss  of  nitrogenous  matter  by  liberating 
the  nitrogen  as  free  nitrogen  gas.  Excrement  always  contains 
them,  but  they  are  more  abundant  in  the  excrement  of  herbivorous 
animals  than  of  carnivorous  animals.  These  bacteria,  in  order  to 
grow  vigorously,  require  some  carbon-holding  food,  and  they  cause 
the  largest  amount  of  denitrification  when  abundantly  supplied 
with  carbohydrates.     Sugars,  starches,  glycerin,   or  organic  acids 


68  NITRIFICATION    AND    DENITRIFICATION. 

may  furnish  this  needed  carbon,  and  the  cellulose  present  in  hay 
or  straw  will  also  furnish  it.  Any  form  of  decaying  matter  that 
contains  great  amounts  of  hay  or  stubble  is  especially  subject 
to  denitrification.  Horse  manure,  containing  as  it  does  large 
amounts  of  hay,  shows  greater  losses  of  nitrogen  than  the  manure 
of  cattle,  which  contains  less  carbonaceous  material. 

The  extent  of  the  actual  losses  caused  by  these  denitrifiers  in 
ordinary  farm  processes  is  not  fully  known.  It  is  certain  that  in 
concentrated  decomposing  solutions  the  action  is  vigorous,  but 
it  is  not  so  great  in  less  concentrated  masses.  In  the  manure  heap 
there  is  always  some  loss  in  this  way,  and  when  great  quantities 
of  manure  are  spread  over  a  plot  of  cultivated  ground,  denitrifica- 
tion doubtless  causes  considerable  loss.  When,  however,  the  manure 
is  applied  in  limited  quantity,  so  that  it  is  mixed  with  a  considerable 
amount  of  soil,  the  evidence  seems  to  show  that  the  losses  are  slight 
if  any.  In  ordinary  soil,  therefore,  denitrification  is  not  a  phenome- 
non of  much  significance.  In  concentrated  manures,  however, 
especially  if  they  contain  much  hay,  it  may  be  great.  One  very 
impoitant  lesson  is  to  be  drawn  from  these  facts.  Nitrates  should 
never  be  mixed  with  manure.  The  nitrates  will  simply  be  thrown 
away,  since  the  denitrification  in  the  manure  heap  will  surely 
reduce  most  of  the  nitrate  to  free  nitrogen,  thus  causing  its  complete 
loss.  Further,  the  denitrification  is  greatest  in  fresh,  concentrated 
manure,  while  it  diminishes  greatly  in  manure  after  it  has  partly 
decayed.  The  denitrifiers  do  not  find  the  partly  decomposed 
organic  substance  favorable  to  their  life,  and  do  not  flourish.  Hence, 
the  use  of  large  amounts  of  partly  rotted  manure  upon  a  soil  is 
possible  without  bringing  about  a  nitrogenous  loss,  while  the  use 
of  the  same  amount  of  fresh  manure  would  be  undesirable. 


CHAPTER  VI. 
THE  MANURE  HEAP  AND  SEWAGE. 

CONTENTS  OF  THE  MANURE  HEAP. 

The  value  of  the  manure  heap  is  recognized  by  every  farmer. 
So  thoroughly  is  this  appreciated  that,  in  some  countries,  the  wealth 
of  the  farmer  is  measured  by  the  size  of  his  manure  heap,  which 
is  commonly  exposed  prominently  in  front  of  his  house.  Every- 
where one  may  measure  quite  accurately  the  thrift  of  a  farmer  by 
an  examination  of  this  somewhat  unsavory  product  of  farm  life, 
and  the  extent  of  his  intelligence  may  likewise  be  gauged  by  the 
care  he  bestows  upon  it.  We  can  readily  understand  its  importance 
when  we  remember  that  in  this  manure  heap  are  going  on,  in  a 
condensed  space,  exactly  the  transformations  of  food  material 
which  we  have  been  considering. 

The  manure  heap  is  always  an  extremely  complex  mixture 
of  organic  substances,  of  nearly  every  conceivable  kind.  It  contains 
great  quantities  of  partly  broken-down  vegetable  tissues,  which  have 
passed  through  the  alimentary  canal  of  the  cattle,  partly  digested. 
It  will  contain  large  or  small  amounts  of  hay  or  straw  derived  from 
bedding  and  from  the  incompletely  digested  food,  especially  if 
horses  contribute  to  its  formation.  It  may  contain  sawdust  or 
some  other  form  of  woody  tissue.  It  will  be  likely  to  contain  more 
or  less  flesh  and  bone  from  dead  animals,  and  will  be  sure  to  contain 
proteids,  albuminoids,  gelatins,  fats,  sugars,  starches,  and,  indeed, 
nearly  all  types  of  organic  matter  produced  by  animals  or  plants, 
all  of  which  will  be  in  various  stages  of  digestion  and  decomposition. 
Lastly,  and  perhaps  most  important,  it  will  contain  much  nitrogen 
in  the  form  of  urea,  in  the  liquid  manure,  which  represents  the 
result  of  the  nitrogenous  metabolism  of  animal  life.  This  liquid 
manure  is  by  far  the  most  valuable  part  of  the  manure,  since  it 

69 


70  THE  MANURE  HEAP  AND  SEWAGE. 

contains  the  nitrogen  which  has  been  actually  metabolized  by  ani- 
mals, and  which  can  now  be  brought  back  readily  into  a  condition 
available  for  plant  life.  The  liquid  manure  contains  three-fourths 
of  the  total  nitrogen  of  the  whole  heap,  and  four-fifths  of  the  total 
potash.  But  farmers  frequently  fail  to  realize  this,  and  allow  this 
material  to  waste  by  soaking  into  the  ground.  In  addition  to  these 
ingredients  manure  always  contains  a  large  amount  of  water  and 
an  unknown  number  of  species  and  varieties  of  bacteria  in  very 
great  abundance. 

In  this  manure  the  bacteria  find  plenty  of  food  and  moisture 
and  their  growth  is  rapid.  There  is  a  great  struggle  for  existence 
among  them  and,  in  the  weeks  of  fermentation,  first  one  and  then 
another  species  may  gain  mastery.  If  the  bacterial  contents  of 
such  a  mass  be  studied  at  intervals,  the  number  and  variety  of  species 
which  are  most  abundant  are  found  to  be  constantly  changing. 
At  first  the  ordinary  intestinal  bacteria  abound;  later  the  putrefactive 
bacteria  become  most  abundant,  and  finally  the  denitrifying  and 
nitrifying  bacteria  are  in  the  majority.  All  of  this  indicates  faintly 
the  wonderful  complexity  of  bacterial  life  and  the  intensity  of  the 
struggle  for  existence  among  the  numerous  species  originally  present 
in  the  manure. 

Losses  from  the  Manure  Pile. — The  result  of  this  bacteria 
growth  is  an  extensive  and  profound  series  of  chemical  changes 
by  which  the  manure  is  profoundly  modified.  These  are  partly 
useful  and  partly  injurious,  but,  taken  as  a  whole,  they  are  neces- 
sary. Most  of  the  material  in  the  manure  is  in  a  form  not  capable 
of  being  used  by  plants,  and  must  be  greatly  transformed  before  it  is 
available  for  vegetation.  The  transformations  are  much  the  same 
as  those  we  have  already  considered  in  the  soil,  but  they  take  place 
under  different  conditions,  which  somewhat  modify  them.  In  our 
study  of  the  subject  it  should  be  borne  in  mind  that  the  most  im- 
portant feature  of  manuring  is  the  furnishing  of  nitrogen  to  the  crops, 
and  the  first  care  should  be  to  protect  this  material  and  avoid  its 
loss. 

The  losses  from  manure  are  due  to  two  causes,  i.  Leaching.  A 
considerable  portion  of  the  nitrogen  is  in  a  soluble  form,  including 


THE    FERMENTATIONS    OF   MANURE.  7I 

all  of  that  in  the  liquid  Inanure.  From  manure  heaped  upon  soft 
ground,  large  amounts  of  this  are  completely  lost  by  draining  away 
or  soaking  into  the  ground.  If  the  manure  is  left  exposed  to  rains 
this  loss  is  greatly  increased.  As  a  result  the  ordinary  manure  heap 
decreases  very  much  in  value  during  the  weeks  or  months  that  it  is 
stored  in  the  pile.  This  part  of  the  loss  can  be  entirely  prevented 
by  storing  the  manure  where  the  liquids  will  not  leach  into  the 
soil.  2.  By  fermentation.  This  subject  requires  a  more  extended 
consideration. 

THE  FERMENTATIONS  OF  MANURE. 

Destructive. — The  first  chemical  changes  which  go  on  are 
those  of  general  decomposition.  An  ammoniacal  fermentation  is 
universal.  The  liquid  manure  is  most  rapidly  decomposed  by  this 
fermentation,  the  substance  undergoing  in  a  very  few  days,  sometimes 
in  a  few  hours,  a  reduction  into  ammonia  compounds,  as  already 
mentioned  above.  This  is  completed  before  the  ammoniacal  fer- 
mentation of  the  other  nitrogen  bodies  has  fairly  begun,  and  suggests 
that  the  proper  method  of  handling  manure  will  be  to  treat  the 
liquid  manure  separately  from  the  solid  portion.  Eventually  the 
nitrogenous  compounds  in  the  solid  manure  will  also  undergo  am- 
moniacal fermentation.  The  starches,  sugars,  cellulose  and  woody 
tissue  undergo  a  decomposition  by  which  COj  is  set  free  and  various 
other  substances  are  left.  The  fats  and  fatty  acids  are  also  decom- 
posed, liberating  CO2  with  other  less  known  bodies.  The  decom- 
position of  the  proteids  liberates  sulphur,  commonly  as  H^S,  and  this 
may  unite  with  water  to  form  sulphuric  acid.  The  sulphuric  acid 
may  combine  with  the  ammonia  to  form  ammonium  sulphate,  or  the 
ammonia  may  combine  with  the  carbon  to  form  carbonates.  A 
large  quantity  of  material  is  lost  from  the  manure  during  these 
changes.  The  loss  includes  carbon  in  large  amount,  a  matter  of  no 
significance,  however,  as  it  has  simply  gone  into  the  air  from  which 
it  can  readily  be  reclaimed  by  plants.  But  the  loss  includes  much 
nitrogen,  and  this  is  a  misfortune,  since  it  is  the  nitrogen  that  the 
farmer  desires  to  keep. 


72  THE  MANURE  HEAP  AND  SEWAGE. 

This  loss  occurs  in  several  ways.  i.  Liberation  of  ammonia. 
Since  the  ammonia  resulting  from  decomposition  is  a  gas,  it  will,  to  a 
considerable  extent,  dissipate  itself  at  once  into  the  atmosphere. 
Such  portions  of  it  as  unite  with  carbon  dioxid  to  form  ammonium 
carbonate  are  less  volatile,  but  this,  too,  is  partly  volatilized.  The 
odor  of  ammonia  common  around  a  manure  heap  plainly  demon- 
strates this  loss.  2.  Denitrification.  If  by  this  term  we  refer  only 
to  the  reduction  of  nitrates  so  as  to  set  nitrogen  free,  the  process  is 
not  very  important  in  a  manure  heap,  since  there  is  present  only  a 
little  nitrate,  none,  indeed,  at  first,  when  the  fermentations  are 
greatest.  If  nitrates  are  present,  denitrification  will  cause  a  loss,  and 
in  the  later  stages  of  the  rotting  of  manure,  after  nitrates  are  formed, 
this  loss  might  be  considerable.  But  it  seems  that  loss  from  this 
cause  is  not  so  great  as  was  formerly  supposed.  3.  Destruction  of 
ammonia.  There  seems  to  be  a  direct  "burning"  of  ammonia 
compounds  in  the  manure  heap  by  which  the  nitrogen  is  set  free 
from  it  as  free  nitrogen.  Little  is  known  concerning  this  factor  at 
present. 

The  extent  of  the  nitrogen  losses  from  these  sources  may  be 
considerable.  Various  estimates  of  the  amount  have  been  made, 
and  it  seems  not  beyond  the  mark  to  say  that,  in  the  ordinary  condi- 
tions on  the  farm,  at  least  50  per  cent,  of  the  nitrogen  is  lost  to  the 
manure.  It  is  sometimes  considerably  more  than  this,  50  per  cent, 
being  a  fair  average.  When  the  farmer  remembers  the  high  cost  of 
nitrogen  fertilizers  he  may  perhaps  realize  the  very  poor  economy  of 
allowing  this  loss  to  continue.  Not  all  the  loss  is  avoidable,  for 
under  the  best  conditions,  perhaps  15  per  cent,  is  lost;  but  even  if  this 
is  true,  35  per  cent,  may  be  saved  by  proper  care.  It  is  possible  for 
a  farmer  to  know  when  this  loss  is  becoming  excessive  by  two  means : 
I.  The  appearance  of  a  strong  odor  of  ammonia  tells  its  own  story; 
and  while  some  such  odor  may  always  be  expected,  a  strong  odor 
indicates  a  too  rapid  loss.  2.  The  heating  of  the  manure  indicates 
rapid  aerobic  fermentations  and  this  is  always  accompanied  by  a 
large  nitrogen  loss.  Properly  kept  manure  will  not  show  a  great 
rise  in  temperature  and  never  a  rapid  one.  The  farmer  may  be 
confident  that  a  noticeable  heating  of  his  manure  pile  means  a  large 


THE    FERMENTATIONS    OF   MANURE.  73 

loss.  Some  manure  he^ts  more  rapidly  than  others,  that  from  the 
horse  being  especially  subject  to  this  destructive  fermentation;  a 
fact  due  partly  to  the  large  amount  of  hay  that  it  contains  and  partly 
to  its  loose  and  porous  nature,  which  allows  a  free  access  of  air.  It 
suffers  more  loss  for  this  reason  than  most  other  types  of  manure — a 
loss  that  may  be  lessened  by  mixing  with  it  some  of  the  moister, 
denser  cow  manure.  Liquid  manure  also  is  subject  to  heavy  losses, 
because  it  so  rapidly  undergoes  the  ammonical  fermentation. 

There  are  two  general  methods  of  controling  and  reducing  these 
losses.  The  first  is  by  chemical  means.  Since  the  ammonia  is 
volatile  and  a  strong  base,  the  addition  to  the  manure  of  some 
chemical  to  conbine  with  it  will  produce  salts  that  will  be  more 
likely  to  be  retained  in  the  manure.  For  this  purpose  quite  a  list 
of  substances  has  been  recommended.  Among  them  are,  gypsum, 
burned  lime,  shell  lime,  lime-stone,  kainii,  superphosphates  and  sul- 
phuric acid.  Each  of  these  has  a  value  when  properly  used,  but 
none  of  them  will  wholly  prevent  nitrogen  loss  and  all  are  some- 
what costly.  The  losses  due  to  ammonia  vaporization  may  be  pre- 
vented by  these  chemicals;  but  the  losses  caused  by  the  liberation 
of  free  nitrogen  cannot  be  checked  by  any  means  short  of  stopping 
bacterial  growth,  and  this  would  check  the  beneficient  as  well  as  the 
injurious  fermentations.  On  the  whole,  the  result  of  experience 
seems  at  present  to  be  against  the  use  of  chemical  means  of 
preserving  manure. 

The  second  method  is  mechanical  and  is  more  efficient.  It  is 
based  upon  the  facts  already  emphasized,  viz.,  that  the  destructive 
fermentations  take  place  most  vigorously  in  the  presence  of  a  large 
supply  of  oxygen  and  that  the  volitilization  is  much  more  rapid 
from  a  partly  dry  than  from  a  wet  mass.  Hence  manure  that  is 
loosely  piled  loses  much  more  nitrogen  than  that  which  is  firmly 
compacted.  The  practice  of  firmly  compacting  manure  into  conical 
heaps  with  smooth  sides  is  best  calculated  to  reduce  the  losses  to  a 
minimum.  Experiment  has  shown  that  a  lot  of  manure  firmly 
compacted  may  lose  15  per  cent,  of  its  nitrogen  during  storage, 
while  a  similar  lot  loosely  stored  loses  35  per  cent.;  a  very  striking 
testimony  to  the  value  of  compacting.  If,  further,  the  manure  be 
7 


74  THE  MANURE  HEAP  AND  SEWAGE. 

Stored  in  cemented  pits,  that  will  prevent  loss  by  draining,  and  if 
the  excess  of  the  liquid  manure  is  caught  in  special  tanks  and 
frequently  spread  upon  the  fields  before  fermentation  has  progressed 
far  enough  to  cause^much  ammoniacal  fermentation,  the  greater 
part  of  the  ordinary  losses  may  be  prevented.  In  thus  storing  the 
manure  it  should  be  kept  moist  by  the  use  of  liquid  manure,  but 
not  allowed  to  become  water  soaked.  It  has  recently  been  shown 
that  the  losses  from  manure  may  be  greatly  reduced  by  spreading 
fresh  manure  upon  the  older  manure  that  has  already  begun  to 
undergo  an  active  fermentation,  probably  because  the  carbonic 
dioxid  evolved  in  the  fermentation  of  the  older  portions  combines 
with  the  ammonia  developed  from  the  newer  portions,  this  checking 
its  dissipation.  The  presence  of  large  amounts  of  hay  in  manure 
makes  it  difficult  to  compact  and,  moreover,  furnishes  large  amounts 
of  fermentable  matter  that  increases  nitrogen  loss.  It  is  therefore 
usually  unwise  to  allow  much  hay  or  straw  to  be  mixed  with  solid 
manure. 

Thus  the  best  methods  of  protecting  manure  from  loss  are:  i. 
The  exclusion  of  air.  2.  The  regulation  of  the  amount  of  moisture. 
3.  The  separation  of  the  excess  of  liquid  manure  from  the  solid  and 
its  distribution  upon  the  soil  at  frequent  intervals.  4.  Prevention 
of  loss  in  liquid  manure  by  leaching.  5.  The  presence  of  some 
already  fermenting  manure  to  furnish  carbonic  dioxid  to  combine 
with  the  ammonia  as  it  is  produced.  6.  The  use  of  some  kind  of 
^^ litter  ^^  to  absorb  the  liquid  manure  and  prevent  its  loss. 

Constructive  Fermentations. — In  order  that  the  manure  may 
become  plant  food  the  end-products  of  decomposition  must  be  built 
up  into  the  form  of  nitrates  by  nitrification.  Nitrification,  however, 
cannot  take  place  in  the  fresh  manure  since  it  contains  too  large 
quantities  of  organic  products,  which,  as  we  have  seen,  prevent  the 
growth  of  nitrifyers. 

Exactly  when  it  begins  is  a  little  uncertain,  but  it  appears  to 
start  only  after  the  high  organic  compounds  have  been  almost 
wholly  broken  up  into  ammonia,  and  the  ammonia  formed  has 
either  united  with  the  acids  to  form  salts  or  has  been  dissipated 
into  the  air.     The  oxidation  of  the  ammonia  salts  into  nitrites  is 


THE    FERMENTATIONS    OF   MANURE.  75 

then  brought  about  by  the  nitrous  bacteria  which  are  not  prevented 
from  growing  by  the  presence  of  free  ammonia.  The  nitric  bacteria 
are,  however,  so  extremely  sensitive  to  ammonia  that  they  cannot 
begin  the  formation  of  nitric  acid  till  ammonia  gas  has  entirely 
disappeared  and  therefore  probably  not  until  decomposition  has 
ceased.  When  the  nitrifying  processes  do  begin,  they  complete  the 
ripening  of  the  manure.  They  oxidize  the  nitrogen  compounds 
which  are  left,  the  ammonia  salts  becoming  first  changed  to  nitrites 
and  then  to  nitrates.  As  this  process  continues  the  manure  is 
more  and  more  filled  with  nitrates  and  therefore  becomes  a  better 
and  better  food  for  plants.  At  last  when  the  process  is  ended  and 
the  manure  is  fully  ripened,  enough  of  nitrogen  is  converted  into 
nitrates  to  furnish  a  most  valuable  supply  of  food  for  vegetation. 

Fresh  and  Ripened  Manure. — The  transformations  which 
we  have  considered  constitute  what  is  called  the  ripenings  rotting, 
or  composting  of  manure.  They  are  clearly  similar  to  those  changes 
already  considered  as  taking  place  in  the  transformations  in  the 
humus,  but  rendered  more  intense  by  the  concentration  of  the 
manure  heap.  It  is  evident  that  manure  is  of  no  value  to  plants 
until  it  has  undergone  these  transformations,  and  equally  evident 
that  the  transformations  may,  and  some  of  them  do,  go  on  in  the 
soil  after  the  manure  is  mixed  with  it  as  well  as  in  the  manure  heap. 
Indeed,  they  will  probably  go  on  better  in  the  soil,  and  in  some 
important  respects  it  is  an  advantage  to  incorporate  the  manure 
with  the  soil  while  fresh  rather  than  to  wait  for  it  to  ripen.  We  have 
noticed  that  the  loss  from  decomposition  and  denitrification  is 
slight  when  these  processes  occur  in  the  soil,  while  they  are  con- 
siderably higher  when  the  ripening  occurs  in  the  concentrated 
manure  heap.  The  loss  is  especially  large  from  the  liquid  manure 
in  warm  weather,  which,  if  kept  in  tanks  or  allowed  to  accumulate 
with  the  manure  pile,  will  undergo  a  very  rapid  ammoniacal 
fermentation  resulting  in  large  losses  of  nitrogen.  If,  however, 
it  is  mixed  at  once  with  the  soil  the  ammonia  is  fixed  as  fast  as  formed 
by  the  soil  ingredients,  is  soon  nitrified,  and  the  loss  is  largely 
prevented.  There  has  thus  come  to  be  recommended  the  practice 
of  spreading  the  manure  upon  the  soil  as  quickly  as  convenient. 


76  THE  MANURE  HEAP  AND  SEWAGE. 

not  allowing  it  to  accumulate,  and  undergo  the  fermentation  that 
inevitable  means  loss  of  nitrogen.  Whether  this  will  always  be 
feasible  will  depend  upon  conditions  on  the  individual  farm;  but 
it  is  certainly  to  be  highly  recommended  in  all  localities  where 
climatic  conditions  and  the  exigencies  of  farm  occupations  make  it 
possible. 

It  is  to  be  borne  in  mind  that  in  using  manure  as  a  fertilizer 
the  soil  receives  advantages  that  are  not  derived  from  mineral 
fertilizers.  Not  only  does  the  manure  contain  a  considerable  list 
of  substances  of  value  to  the  crops,  present  in  small  quantities 
only,  and  not  present  in  mineral  fertilizers,  but  manure  contains 
considerable  organic  material  in  a  partly  decomposed  condition 
that  aids  in  forming  a  permanent  humus.  The  texture  of  the  soil 
is  improved  by  this  so  that  the  final  result  is  a  soil  superior  to  that 
containing  only  mineral  fertilizers.  The  soil  thus  treated  becomes 
more  tenacious,  richer,  washes  less  with  rains  and  is  generally  to 
be  preferred.  Mineral  fertilizers,  with  the  exception  of  nitrates 
may  be  mixed  with  manure.  The  nitrates,  if  thus  mixed,  would 
be  lost  by  denitrification. 

SALTPETER  PLANTATIONS. 

These  nitrifying  forces  are  not  confined  to  the  soil,  but  may 
occur  in  other  localities,  always  resulting  in  the  production  of  nitrates. 
Before  the  discovery  of  the  nitrate  beds  of  South  America  it  was  the 
custom  of  agriculturists  to  prepare  their  own  nitrates  by  a  simple 
process,  not  then  understood,  but  now  known  to  be  due  to  nitrifying 
bacteria.  The  places  where  nitrates  were  thus  formed  were  called 
saltpeter  plantations,  and  the  saltpeter  was  produced  by  exactly  the 
processes  we  have  already  considered.     The  method  was  as  follows : 

Masses  of  chalky  soil  were  mixed  with  various  organic  bodies 
and  the  whole  heaped  into  a  pyramidal  pile,  rendered  somewhat 
porous  by  the  admixture  of  brushwood.  The  heap  was  still  further 
furnished  with  fermentable  nitrogen  by  frequently  watering  it  with 
liquid  manure.  In  this  heap  occurred  the  various  kinds  of  nitrogen 
decomposition  already  mentioned,  and  later  the  nitrification  process 


THE    COMPOST    HEAP. 


77 


began.  The  result  was  Ihat  nitrates  were  formed  in  the  interior  of 
the  heap  in  large  quantity.  Eventually  the  nitrates  were  extracted 
by  water  and  converted  into  nitrate  of  potassium  by  the  addition  of 
some  potassium  salt. 

This  method  of  making  saltpeter  was  discovered  before  science 
had  any  idea  of  the  real  nature  of  the  process,  and  it  was  a  practical 
means  of  utilizing  a  part  of  the  nitrogen  in  the  organic  substances 
derived  from  animals  and  plants.  Whether  it  was  the  most  efficient 
means  or  more  useful  than  the  simple  compost  heap  and  manure 
pile  can  hardly  be  stated. 

Saltpeter  plantations  have  gone  out  of  existence  since  the  in- 
troduction of  Chilian  saltpeter.  It  is  probable  that  the  nitrate  beds 
of  Chili  are  the  remains  of  some  old  inland  arms  of  the  sea  where 
great  growths  of  seaweeds  accumulated  which,  after  the  drying  of 
the  inland  sea,  were  converted  into  nitrates  by  the  processes  of 
decomposition  and  nitrification  due  to  bacterial  action. 

Nitrates  are  formed  upon  the  walls  of  closets  and  stables  where 
ammonia  fumes  are  abundant.  On  such  walls  may  frequently  be 
seen  a  snow-white  mass  consisting  of  calcium  nitrate.  It  is  the  result 
of  nitrification  of  the  ammonia  which  unites  with  oxygen  and  pro- 
duces nitric  acid.  The  acid  combines  with  the  calcium  present  in 
the  brick- work  to  form  calcium  nitrate.  The  action  is  an  undesirable 
one  from  the  standpoint  of  the  persistence  of  the  walls,  since  it  pro- 
duces a  corroding  action  tending  to  weaken  the  structure.  It  may 
be  readily  prevented  by  sprinkling  the  walls  with  a  strong  solution  of 
some  powerful  antiseptic,  such  as  formalin  or  corrosive  sublimate. 

THE  COMPOST  HEAP. 

It  is  evident  that  in  a  compost  heap  there  must  be  going  on  a 
series  of  similar  bacterial  transformations.  By  proper  means  the 
farmer  may  make  use  in  his  soil  of  almost  any  organic  material 
which  contains  nitrogen  or  the  minerals  needed  for  his  crops. 
Vegetable  tissues  of  all  sorts  contain  more  or  less  nitrogen  and  may 
readily  be  brought  under  the  influence  of  the  bacteria  which  are 
able  to  reduce  them  to  plant  foods.     A  valuable  source  of  such 


78  THE  MANURE  HEAP  AND  SEWAGE. 

material  may  be  obtained  from  seaweeds,  if  they  are  at  hand.  In- 
deed, any  abundant  vegetable  substances  may  thus  be  heaped  into  a 
mass,  and,  if  moistened  sufficiently  by  rains,  the  bacteria  will  be 
sure  to  work  within  it,  gradually  transforming  the  nitrogens,  and 
converting  them  finally  into  nitrates  for  plant  food.  Into  his 
garbage  heap,  then,  the  farmer  may  throw  all  sorts  of  organic 
debris,  animal  or  vegetable,  with  the  confidence  that  his  bacterial 
aids  will  in  time  place  the  nitrogenous  material  at  his  service  as  a 
fertilizer.  Thus,  by  the  aid  of  his  invisible  allies  the  agriculturist 
will  be  able  to  make  use  of  the  wastes  on  his  farm  and  in  time  return 
to  his  soil  a  considerable  portion  of  the  nitrogen. 

SEWAGE  AND  ITS  TREATMENT. 

Composition  of  Sewage. — By  sewage  we  ordinarily  understand 
the  material  which  collects  in  the  sewerage  system  of  our  larger  com- 
munities and  which  has  no  exact  counterpart  on  the  farm.  It  al- 
ways contains  the  products  of  the  life  of  men  and  animals,  which  are 
no  longer  useful;  also  large  quantities  of  both  animal  and  vegetable 
foods  which  have  passed  through  the  alimentary  canals  of  men  and 
animals  unassimilated.  It  contains  a  large  amount  of  urea  which 
has  come  from  the  animal  metabolism;  and  also  woody  matter, 
cellulose,  fat,  starch,  and  an  indefinite  series  of  other  organic 
bodies.  Almost  anything  which  enters  the  city  may  find  its  way 
eventually  into  the  sewers  where,  mixed  with  large  quantities  of 
water,  it  contributes  to  the  sewage.  The  sewage  thus  contains 
exactly  the  same  sort  of  material  as  that  found  in  the  manure  heap 
and  the  compost  pile.  Evidently  the  problem  of  the  various  steps 
of  decomposition  of  this  material  will  be  nearly  identical  with  that 
already  considered. 

TREATMENT  OF  CITY  SEWAGE. 

As  cities  have  grown,  the  matter  of  disposing  of  their  sewage  be- 
comes more  and  more  difficult.  In  small  communities  the  digging 
of  cess-pools  is  satisfactory;  but  as  larger  numbers  of  people  con- 


TREATMENT    OF    CITY    SEWAGE.  79 

gregate  together,  this  method  becomes  objectionable,  and  finally 
impossible.  The  treatment  of  city  sewage  has  become  a  problem 
involving  the  ingenuity  of  the  expert  sanitary  engineer.  It  cannot 
be  said  that  any  wholly  satisfactory  method  has  yet  been  devised 
for  handling  this  difficult  problem. 

In  the  first  place  it  is  evident  that  sewage  contains  material  that 
is  very  valuable  if  it  can  be  used  upon  the  soil.  The  large  amounts 
of  nitrogen  in  the  urea  alone  from  a  large  city  would  be  worth  mil- 
lions of  dollars  yearly  if  it  could  be  utilized.  The  nitrogen  was  taken 
originally  from  the  soil  by  the  crops,  and  the  continued  fjertility  of 
the  soil  is  dependent  upon  its  being  in  some  way  replaced.  It  re- 
quires no  argument  to  show  the  wastefulness  of  throwing  this  valu- 
able material  away  without  attempting  to  utilize  it.  In  China 
careful  attention  is  paid  to  prevent  the  loss  of  such  material,  and  as  a 
result  the  soil  remains  fertile;  while  in  our  country  a  constantly 
decreasing  fertility  has  followed  the  practice  of  wasting  it.  The  only 
methods  yet  devised  of  utilizing  city  sewage  on  a  large  scale  is  by 
what  is  called  sewage  farming. 

Sewage  Farming. — This  method  of  disposing  of  sewage  has 
been  established  in  the  last  thirty  years  as  a  means  of  at  once  dis- 
posing of  and  utilizing  the  sewage  of  large  cities.  These  farms, 
necessarily  located  as  near  as  possible  to  the  city,  receive  its  sewage 
and  distribute  it  over  the  fields  by  conduits,  thus  furnishing  the  crops 
at  the  same  time  with  nourishment  and  water.  Upon  such  soils 
crops  are  raised,  mostly  garden  crops,  since  these  are  sure  of  a  ready 
market  in  the  city.  This  plan  of  utilizing  sewage  has  been  very 
vigorously  urged,  and  many  such  farms  have  been  organized  in 
England,  in  continental  Europe,  and  some  in  this  country.  Enor- 
mous sewage  farms  are  cultivated  near  the  cities  of  Paris  and  Berlin, 
the  latter  city  having  thousands  of  acres  under  cultivation.  In  some 
of  the  arid  western  sections  of  United  States,  where  water  is  es- 
pecially valuable,  sewage  farming  has  also  become  very  profitable. 

There  can  be  no  doubt  that  this  method  of  disposing  of  sewage  is, 
theoretically,  the  proper  one.  It  has  two  distinct  advantages: 
I.  Economic.  It  puts  back  into  the  soil  the  great  quantities  of 
nitrogen  and  other  materials  taken  from  it  by  the  crops.     2.  Sani- 


8o  THE  MANURE  HEAP  AND  SEWAGE. 

tary.  The  sewage  ingredients, "  after  being  incorporated  into  the 
soil,  undergo  the  various  types  of  bacterial  decompositions  which 
have  been  described  in  recent  chapters.  The  organic  compounds 
are  decomposed  by  bacterial  action,  a  part  of  the  resulting  decom- 
position products  going  into  the  air  as  gas,  and  a  part  being  built  up, 
in  the  soil,  into  nitrates,  to  feed  the  growing  crops.  The  offensive 
waste  material  is  thus  disposed  of  by  being  converted  into  inoffen- 
sive and  useful  products.  Sewage  farms  thus  prevent  the  sewage 
contamination  of  streams,  harbors,  and  seaside  resorts.  This  in 
itself  is  sufficient  to  make  this  method  of  disposal  a  desirable  one. 

But  the  appearance  of  certain  practical  difficulties  has  prevented 
a  wide  development  of  this  system.  It  is,  of  course,  impossible  to 
expect  farmers  to  adopt  this  system  unless  it  is  profitable,  and,  un- 
fortunately, it  frequently  proves  that  such  sewage  farms  are  run  at 
a  loss  instead  of  a  gain.  To  be  sure,  in  some  places  very  favorable 
returns  have  been  yielded,  and  largely  increased  crops  have  been 
reported.  But  in  other  places  unfavorable  reports  have  been  made, 
and  at  the  present  time  sewage  farming  is  not  increasing.  Since 
the  objections  to  it  are  purely  practical,  they  may  in  time  be  over- 
come. The  chief  objections  are  as  follows:  i.  It  is  unhealthful  to  ir- 
rigate garden  crops  with  a  sewage  that  is  sure  to  contain  disease 
germs.  This  objection  applies  chiefly  to  vegetable  products  which 
are  eaten  without  cooking,  like  lettuce,  celery,  etc.  2.  Land  near 
the  large  cities  is  usually  too  valuable  to  be  used  for  farming  proc- 
esses. If  the  farm  is  at  some  distance  from  the  city  the  expense  of 
carrying  the  sewage  to  it  becomes  so  great  as  to  make  the  under- 
taking a  losing  instead  of  a  profitable  one.  3.  Only  a  fairly  porous 
and  partly  sandy  soil  can  absorb  the  quantity  of  sewage  necessary 
for  the  disposal  of  the  product.  In  order  that  the  soil  may  absorb- 
it,  the  sewage  must  be  decomposed  and  nitrified  by  bacteria.  If 
this  nitrification  does  not  occur,  the  soil  becomes  clogged  with  the 
sewage  products.  Hence,  if  the  soil  is  heavy  and  contains  much 
clay  it  cannot  be  used  for  sewage  farming.  While  sewage  farms 
are  successful  and  profitable  in  the  sandy  soils  around  Berlin,  they 
are  not  possible  in  many  another  locality  where  the  soil  is  of  a  dif- 
ferent texture.     4.  The  extreme  dilution  of  the  sewage  makes  it 


t 


TREATMENT    OF    CITY    SEWAGE.  8 1 

mpractical,  or  even  impossible,  to  use  it  in  some  localities.  The 
il  can  absorb  only  a  certain  amount  of  water  without  becoming 
too  wet  for  the  raising  of  crops.  In  regions  where  rains  are  common, 
all  the  water  is  furnished  by  rain  that  can  be  handled  by  the  soil, 
unless  it  is  very  sandy,  so  that  the  addition  of  much  more  water 
would  spoil  the  crops.  In  many  localities,  especially  in  the  United 
States,  the  sewage  from  a  city  is  very  dilute.  We  are  very  ex- 
travagant in  the  use  of  water,  and  a  given  quantity  of  American 
sewage  contains  much  less  fertilizing  material  than  the  same  amount 
of  German  sewage.  American  sewage  cannot  be  valued  at  more 
than  one  cent  a  ton,  and  it  is  impossible  for  any  but  tha  most  sandy 
soils  to  absorb  to  advantage  such  great  quantities  of  water,  for  the 
minute  quantity  of  nourishing  matter  it  contains. 

These  are  some  of  the  reasons  why  sewage  farming  is  not 
generally  successful,  and  there  is  little  to  hope  for  along  these  lines 
until  some  new  methods  can  be  devised  for  making  the  fertilizing 
ingredients  of  sewage  more  easily  available  and  more  profitable. 
It  must  be  recognized,  however,  that  this  end  is  to  be  desired,  since 
surely  there  ought  to  be  some  method  of  saving  the  enormous  loss 
that  comes  from  waste  sewage.  It  can  at  present  be  profitably 
undertaken  only  in  dry  regions,  or  where  a  sandy  soil  can  absorb 
large  amounts  of  water. 

Sewage  Disposed  of  as  a  Waste  Product. — For  the  reasons 
just  given  it  will  be  understood  why  sewage  has  come  to  be  regarded 
as  a  waste  product,  to  be  disposed  of  as  inexpensively,  but  as  ef- 
ficiently as  possible,  the  desire  being  to  destroy  it  and  not  utilize  it. 

Chemical  Treatment. — The  first  method  used  was  to  treat  the 
sewage  with  chemicals  that  partly  purify  it.  This  is  an  expensive 
method,  troublesome  and  unsatisfactory,  and  although  used  for  a 
few  years,  it  has  been  replaced  quite  generally  by  the  bacterial 
method  of  treatment,  which  is  the  one  most  commonly  adopted 
to-day  by  communities  that  need  to  find  some  method  of  sewage 
disposal. 

The  Bacterial  Treatment  of  Sewage. — This  method  is  based 
upon  exactly  the  same  bacterial  activities  that  we  have  been  con- 
sidering in  recent  chapters,  the  sewage  being  treated  in  a  manner 


82  THE  MANURE  HEAP  AND  SEWAGE. 

that  hastens  the  decomposition  power  of  the  bacteria  so  that  they 
will  rapidly  destroy  the  organic  products  in  the  sewage.  The 
method  has  not  been  devised  by  any  one  person,  but  has  been  the 
result  of  observations  and  experiments  of  several,  extending  over 
many  years,  and  finally  crystallized  into  practical  results. 

The  bacterial  treatment  of  sewage  depends  upon  the  destructive 
action  of  the  decomposition  and  putrefactive  bacteria.  Putre- 
factive bacteria  decompose  all  kinds  of  organic  bodies,  both  the 
nitrogenous  and  those  purely  carbonaceous.  Most  of  the  solid 
matter  in  the  sewage  is  composed  of  these  organic  bodies,  and  it  is 
evident  that  if  the  sewage  can  be  induced  to  undergo  a  thorough 
decomposition  under  the  action  of  microorganisms,  this  will  produce 
a  great  effect  upon  the  composition  of  solid  matters  present. 
Almost  all  of  them  will  be  reduced  to  simpler  compounds.  The 
carbonaceous  material  will  be  reduced  eventually,  if  the  process 
is  complete,  into  CO2  and  water,  with  the  liberation  of  hydrogen 
or  perhaps  marsh  gas  (CHJ.  Such  gases  would  leave  the  liquid 
and  join  the  atmosphere.  The  nitrogenous  material  would  suffer 
the  decomposition,  resulting  in  the  production  of  ammonia;  and 
denitrification,  which  would  be  sure  to  occur,  would  still  further 
reduce  this  to  free  nitrogen.  Such  gases  also  would  be  sure  to  join 
the  atmosphere  unless  held  in  solution  in  the  liquids.  In  short, 
the  putrefactive  processes,  which  in  the  manure  heap  produce  a 
loss  deprecated  by  the  agriculturist,  would  produce  here  exactly 
the  result  which  the  sanitary  engineer  desires  to  reach,  a  destruction 
and  dissipation  of  organic  material. 

Such  changes  will  take  place  as  readily  in  sewage  as  in  manure 
or  in  the  soil.  Indeed,  observation  and  analysis  show  that  they 
commonly  take  place  much  more  rapidly.  In  the  first  place, 
the  organic  matter  to  be  acted  on  is  generally  in  a  soluble  or  partly 
dissolved  condition,  and  very  easily  acted  upon  by  bacteria. 
Secondly,  the  great  abundance  of  water  facilitates  the  action,  for 
bacteria  require  an  abundance  of  water  for  their  best  growth.  Thirdly, 
the  bacteria  are  present  in  extreme  abundance.  All  sewage  con- 
tains bacteria  in  large  numbers,  although  naturally  the  number 
varies.     A  common  sewage  contains  from  7,000,000  to  10,000,000 


TREATMENT    OF    CITY    SEWAGE.  8^ 

bacteria  per  c.c.  Among"  these  bacteria  are  always  large  numbers 
of  the  various  decomposition  bacteria,  ready  to  seize  upon  the 
organic  material  and  decompose  it.  Such  sewage,  if  left  to  itself, 
will  undergo  a  rapid  and  quite  complete  decomposition,  which 
results  in  reducing  large  quantities  of  matter  to  a  gaseous  state. 
Other  parts  are  rendered  perfectly  soluble  and  are  completely 
dissolved  in  the  water,  so  that  the  water  of  the  sewage  is  left  free 
from  putrescible  matter. 

To  bring  about  this  result  two  different  methods  of  treatment 
have  been  adopted,  sometimes  used  together  and  sometimes  sepa- 
rately, each  of  which  has  several  modifications. 

The  Septic  Tank. — This  is  a  method  of  making  use  of  the  anaerobic 
bacteria  which  decompose  products  rapidly,  but  incompletely.  The 
septic  tank  is  a  large  closed  chamber,  perhaps  below  the  surface  of 
the  ground,  and  closed  upon  all  sides  and  the  top,  with  simply  a  vent 
pipe  extending  from  the  top  to  allow  the  escape  of  gases.  The 
sewage  is  passed  into  one  end  of  the  tank  in  a  somewhat  slow  but 
constant  stream,  and  the  cavity  of  the  tank  is  so  divided  by  partitions 
as  to  insure  a  slow  uniform  passage  of  the  sewage  through  the  tank, 
and  a  final  exit  at  the  other  end  by  an  effluent  pipe.  The  flow  is 
regulated  so  that  each  particle  of  sewage  remains  in  the  tank  from 
twenty-four  to  forty-eight  hours. 

During  this  slow  flow  through  the  tank  bacterial  action  is 
vigorous,  and  it  is  chiefly  the  anaerobic  bacteria  that  develop, 
since  the  closed  tank  allows  little  oxygen  to  enter.  Furthermore, 
a  heavy  scum  usually  grows  on  the  surface  that  prevents  the  excess 
of  oxygen.  In  these  anaerobic  conditions,  therefore,  decomposition 
proceeds  rapidly,  the  organic  bodies  becoming  partly  broken  down. 
Gas  is  evolved  in  quantity  and  bubbles  up  through  the  liquid  to 
find  exit  from  the  tank  by  special  vents.  The  gases  represent  the 
partial  destruction  of  the  organic  matters  in  the  sewage,  and  as 
fast  as  they  are  evolved  the  organic  ingredients  in  the  sewage 
disappear  and  the  sewage  becomes  clearer.  When  it  leaves  the 
outlet,  after  flowing  through  the  tank,  it  is  much  purer  than  when 
it  entered,  and  may  then  be  discharged  into  streams  without 
greatly  contaminating  them,  if  the  process  has  been  efficient.    The 


84  THE    MANURE    HEAP    AND    SEWAGE. 

evolved  gases  are  only  partly  oxidized  and  are  sometimes  collected 
and  burned  into  the  final  condition  or  CO^,  etc.  The  fermentation, 
being  of  the  nature  of  putrefaction,  gives  rise  to  unpleasant  odors, 
since  the  gases  contain  various  compounds  of  sulphur  and  phosphorus 
that  are  only  partly  oxidized. 

The  Filter  Bed  and  Contact  Bed. — These  two  methods,  though 
differing  in  detail,  are  identical  in  principle,  and  both  are  designed 
to  stimulate  the  activities  of  the  aerobic  bacteria.  In  the  filter 
beds  the  sewage  is  received  upon  great  open  beds,  the  bottoms  of 
which  are  made  of  masses  of  coke,  broken  stone,  clinkers,  sand, 
etc.,  arranged  in  layers  of  different  degrees  of  fineness,  the  finest  at 
the  top.  Through  these  the  sewage  filters  and  appears  below, 
greatly  purified.  It  was  at  first  supposed  that  the  process  was  a 
mechanical  filtering  through  the  sand,  but  it  is  now  known  that  the 
mechanical  filtering  has  little  to  do  with  it.  The  contact  beds  are 
similar,  large,  open  beds,  filled  with  coarse  coke,  clinkers,  or  other 
material,  but  not  arranged  for  filtering.  The  sewage  is  conducted 
upon  these  beds,  allowed  to  remain  there  for  a  few  hours,  and  then 
withdrawn  to  be  replaced  by  more  sewage.  Although  no  filtering 
takes  place,  this  sewage  is  purified  by  its  sojourn  in  the  contact  bed. 

In  both  cases  the  primary  action  is  that  of  aerobic  bacteria, 
aided,  doubtless,  by  direct  chemical  activities  of  the  oxygen  of  the 
air.  The  bacteria  rapidly  cause  the  decomposition  of  the  organic 
products,  and  the  decomposition  is  more  complete  than  in  the 
septic  tank,  so  that  simple  gases,  like  CO^  and  N  are  evolved. 
The  gases  are  no  longer  oxidizable.  The  action  on  the  sewage  is 
made  more  complete  and  efficient  if  the  sewage  be  first  passed  through 
the  septic  tank  and  then  over  a  contact  or  filter  bed. 

Sprinkling,  Trickling,  or  Percolating  Filters. — This  represents  a 
third  slightly  different  method  of  bacterial  purification  of  sewage.  A 
mass  of  stones  or  some  other  favorable  material,  broken  into 
fragments  not  less  than  half  an  inch  in  diameter,  is  spread  in  a 
layer  several  feet  thick.  The  sewage  is  then  sprayed  or  sprinkled 
upon  this  mass,  and  it  slowly  trickles  through  the  rock  layer.  The 
broken  rock  fragments  are  so  coarse  that  there  is  no  filtering  action, 
but,  as  the  sewage  slowly  trickles  downward,  it  is  acted  upon  by 


TREATMENT    OF    CITY    SEWAGE.  85 

chemical  and  bacterial  kgents,  so  that  it  flows  out  below  quite 
changed  in  its  nature.  Sewage  treated  in  this  way  does  not  look 
clear  after  treatment,  but  the  organic  products  in  it  have  undergone 
a  change  that  makes  them  non-putrescible,  and  they  will  not  undergo 
any  further  putrefactive  changes.  This  method  of  treating  sewage 
is  recent  and  as  yet  not  fully  understood. 

Effect  on  Organic  Products. — As  a  result  of  these  two  types  of 
decomposition  the  various  organic  bodies  in  the  sewage  are  very 
largely  destroyed  by  processes  similar  to  those  that  occur  in  the 
manure  heap.  Various  gases  are  liberated  (HN3,  N,  CO2,  CH^, 
H2S,  etc.),  and  the  total  amount  of  solid  matter  is  thus  greatly  re- 
duced. Later  in  the  process,  especially  in  the  contact  beds  where 
oxygen  is  abundant,  a  vigorous  oxidation  of  the  nitrogen  compounds 
begins  {nitrification)  which  results  in  the  formation  of  nitrates. 
These  nitrates  are,  however,  thoroughly  soluble  and  become  at  once 
dissolved  in  the  water  of  the  sewage,  which  consequently  clears  up. 
In  this  way  nearly  all  of  the  nitrogen  which  was  held  in  high  com- 
pounds in  the  original  sewage,  has  either  become  dissipated  into  the 
air  as  ammonia  or  free  nitrogen,  or  has  become  converted  into 
nitrates  and  has  dissolved  in  the  water  to  form  a  clear  solution  which 
is  not  objectionable  when  discharged  into  streams. 

This  whole  topic  is  only  a  part  of  the  general  subject  of  the 
transformation  of  nitrogen.  Whenever  nitrogenous  matter  is 
mixed  with  water  and  allowed  to  stand  for  a  time,  decomposition 
changes  begin  which  result  in  a  more  or  less  complete  destruction  of 
the  compounds.  This  occurs  in  the  soil,  in  the  manure  heap,  in  the 
privy  vault,  in  the  sink  drain  or  in  sewage,  the  phenomena  being 
fundamentally  the  same  in  all  cases,  although  differing  in  details 
with  differences  in  the  kind  of  compounds  present,  the  amount  of 
water,  the  temperature,  the  access  of  oxygen,  the  species  of  bacteria 
present,  and,  doubtless,  other  factors.  It  results  in  a  purification  of 
the  soil  or  a  purification  of  sewage  from  similar  reasons. 

Effect  on  Bacteria. — It  might  be  supposed  that  the  bacterial 
treatment  would  increase  the  number  of  bacteria  in  the  sewage. 
The  rapid  destruction  of  organic  matter  certainly  points  to  active 
bacterial  growth  and  we  should  expect  to  find  bacteria  more  abun- 


86  THE  MANURE  HEAP  AND  SEWAGE. 

dant  at  the  end  than  at  the  beginning  of  the  treatment.  But  for 
reasons  as  yet  little  understood,  the  reverse  is  the  case.  The 
number  of  bacteria  in  the  treated  sewage  appears  to  be  always  less 
than  in  the  raw  sewage.  The  amount  of  reduction  in  bacteria  is  by 
no  means  constant.  Sometimes  it  is  comparatively  small.  In  a 
series  of  tests  upon  the  sewage  of  London,  treated  in  this  way,  a 
reduction  of  only  about  32  per  cent,  was  found  (7,000,000  to  5,000,- 
000).  In  other  cases  the  reduction  is  greater,  and  sometimes  there 
is  found  a  number  as  high  as  9,000,000  per  c.c.  in  the  raw  sewage, 
and  only  from  5,000  to  10,000  in  the  treated  product.  Something 
evidently  is  at  work  destroying  the  bacteria,  but  its  efficiency  varies 
widely  in  different  instances. 

Whether  these  methods  of  treating  sewage  destroy  its  dangerous 
nature  as  well  as  its  offensiveness  is  not  easy  to  answer.  The 
danger  in  sewage  comes  primarily  from  the  disease  bacteria  it  may 
contain,  foremost  among  which  is  the  typhoid  bacillus.  The 
bacterial  treatment  greatly  reduces  the  number  of  bacteria,  but  does 
not  by  any  means  eliminate  them.  Does  it  eliminate  the  disease 
germs?  So  far  as  evidence  goes  to-day  it  seems  that  the  typhoid 
bacillus  is  eliminated  by  the  treatment,  and  the  effluent  from  such 
beds  fails  to  show  typhoid  bacteria,  even  when  they  have  been  pur- 
posely put  in  the  sewage.  Bacteriologists  who  have  had  confidence 
in  the  efficacy  of  the  purification  have  not  hesitated  to  drink 
freely  of  the  water  from  such  a  sewage  filter  bed.  It  is  certain, 
therefore,  that  the  treatment  greatly  improves  the  healthfulness 
of  the  sewage.  But  that  it  removes  all  danger  from  it  cannot  be 
positively  stated. 

Such  a  disposal  of  sewage  means,  of  course,  a  complete  loss  of  the 
nitrogenous  material,  for  no  method  is  adopted  for  utilizing  the 
wasted  nitrates.  But  this  fact  is  no  longer  regarded  so  seriously  as 
it  was  a  few  years  ago.  We  have  learned  that  there  are  efficient 
forces  in  nature  for  bringing  back  from  the  atmosphere  the  nitrogen 
dissipated  from  the  soil,  and  it  is  a  matter  of  less  significance  to  throw 
away  the  sewage  nitrogen  than  it  appeared  to  be  when  the  only 
known  source  of  nitrogen  was  supposed  to  be  the  fixed  nitrogen  of 
the  soil.     Since  the  soil  can  readily  replace  its  lost  nitrogen  through 


TREATMENT  OF  FARM  SEWAGE. 


TREATMENT    OF    FARM    SEWAGE.  87 

the  agency  of  certain  species  of  bacteria  (see  Chapter  VII),  it  is  no 
\^m  serious  matter  if  some  of  the  nitrogen  is  thrown  away. 

L. 

^^m  Upon  the  ordinary  farm  the  sewage  problem  is  rarely  of  any  im- 
portance, because  of  the  small  amount  of  material.  The  wastes 
which  form  the  sewage  in  the  city  are  kept  separate  on  the  farm  and 
are  not  all  treated  alike.  Part  goes  to  the  manure  or  compost  heap, 
and  later  is  returned  to  the  soil  with  the  manure.  Part  goes  to  the 
privy  vault  and  is  handled  like  manure;  while  still  another  part 
drains  from  the  sink  and  is  generally  allowed  to  waste  itself  on  the 
ground.  A  considerable  portion  of  city  sewage,  like  the  refuse 
from  factories,  etc.,  has  no  counterpart  on  the  farm. 

Nothing  further  need  be  said  concerning  the  first  of  these 
portions.  The  contents  of  the  privy  vault  have  practically  the 
same  relations  to  bacterial  decomposition  and  denitrification  as 
manure,  and  should  be  handled  in  essentially  the  same  manner. 
It  is  always  emphatically  necessary,  however,  to  remember  that 
the  contents  of  the  privy  vault  are  far  more  likely  to  contain  patho- 
genic bacteria  than  is  barnyard  manure,  and  it  should,  consequently, 
be  much  more  carefully  handled.  That  such  material  has  been 
the  means  of  distributing  typhoid  fever  in  many  cases  is  surely 
demonstrated.  The  bacilli  of  this  disease  are  voided  by  the  patient 
in  the  excreta,  and  are  thus  sure  to  find  their  way  into  the  vault, 
to  be  subsequently  distributed  over  the  fields,  where  they  may 
percolate  through  the  soil  and  pollute  streams  and  wells.  The 
contents  of  the  privy  vault  should  never  be  left  in  position  where 
it  can  possibly  pollute  the  water  of  either  brook  or  well.  Precaution 
ns  should  also  be  taken  to  prevent  its  distribution  around  the  farm  by 
means  of  soiled  boots  or  tools  which  have  been  used  in  handling 
it.  There  is  much  more  likelihood  of  finding  pathogenic  bacteria 
in  human  excrement  than  in  that  of  domestic  cattle,  and  the 
disease  germs  thus  found  are  far  more  likely  to  be  injurious  to 
human  health.  Evidently  the  farmer  should  exercise  much  more 
care  in  disposing  of  the  contents  of  his  privy  vault  than  in  the  use  of 


88 


THE  MANURE  HEAP  AND  SEWAGE. 


his  barnyard  manure,  and  the  constant  addition  of  lime  thereto 
is  certainly  to  be  most  thoroughly  recommended.  In  other  respects 
this  material  has  exactly  the  same  relations  to  decomposition  and 
reconstructive  processes  as  barnyard  manure. 

The  portion  of  sewage  which  comes  from  the  wash-water  of 
the  sink  or  the  dairy  on  the  ordinary  farm  is  so  small  that  it  may 
commonly  be  left  to  care  for  itself.  The  amount  of  solid  material 
in  such  water  is  slight,  and  it  can  be  allowed  to  run  out  on  the  soil 
where,  generally,  it  is  rapidly  absorbed  and  decomposed  without 


SEPTIC  TANK 


Fig.  19. — Diagram  showing  the  method  of  applying  the  septic  tank  to  a  farm  house. 

any  undue  pollution.  The  organic  matter  undergoes  the  same  type 
of  decomposition  as  that  to  which  all  organic  bodies  are  subjected 
under  the  influence  of  bacteria,  and  becomes  eventually  converted 
into  plant  food  and  incorporated  into  soil.  The  drainage  which 
comes  from  the  large  dairy  or  creamery  may  be  too  much  to  be 
disposed  of  by  such  a  simple  manner.  In  this  case  some  means 
must  be  adopted  for  its  disposal.  The  problem  thus  presenting 
itself  is  precisely  the  same  as  that  presented  to  the  city  for  disposing 
of  its  sewage,  and  the  same  means  are  to  be  used  in  each  case. 
The  time  is  coming,  and,  in  some  places,  has  arrived,  when 
it  is  necessary  to  find  a  plan  for  disposing  of  the  sewage  on  the  ordi- 
nary farm  in  some  other  way  than  by  emptying  it  into  a  stream. 


TREATMENT    OF    FARM    SEWAGE. 


89 


It  is  very  easy  for  the  farm  to  make  use  of  the  principles  of  a  septic 
tank  in  caring  for  and  even  utilizing  its  sewage.  Fig.  19  shows 
diagrammatically  a  means  of  accomplishing  it  efficiently  and  at 
comparatively  small  expense.  The  diluted  sewage  from  the  house 
is  conducted  to  a  tank  sunk  in  the  ground  at  any  convenient  distance. 
The  tank  should  be  of  such  a  size  that  it  will  hold  the  entire  sewage 
for  twenty-four  hours.     If  each  person  uses  twenty  gallons  of  water 


Profee'ii^  etc. 


jprhteids 


/reeMtpef^eir 


Mtrcus 
/tacter/a^ . .      ^^ 

/    '>^ 

MtrlcMcterJa/ 
Mtrates  f/nscllj 

Fig.  20. — The  nitrogen  cycle. 


-/i^u/n^s 


per  day,  the  tank  for  a  household  of  ten  should  be  three  feet  deep, 
two  feet  wide  and  six  feet  long.  It  must  be  covered  so  as  to  exclude 
air  and  light,  and  the  sewage  must  flow  slowly  and  quietly  through 
the  tank,  thus  making  it  a  septic  tank.  The  discharge  frcrm  the 
tank  is  best  received  into  a  second  tank  from  which  it  can  be 
conducted  to  a  stream  or  upon  the  garden  for  fertilizing  and 
irrigating  it. 


CHAPTER  VII. . 
RECLAIMING  LOST  NITROGEN. 

THE  LOSS  OF  NITROGEN. 

In  spite  of  these  transformations  of  nitrogenous  compounds, 
and  partly  because  of  them,  there  is  a  constant  loss  of  nitrates  from 
the  soil.     This  loss  is  from  the  following  sources: 

1.  Sewage. — Any  animal  or  vegetable  material  that  falls  into 
the  streams  will,  unless  dissipated  into  the  air,  be  carried  to  the 
ocean.  Much  nitrogen  is  brought  to  the  city  where  it  is  used  as 
human  food  and,  as  sewage,  carried  to  the  river  and  perhaps  to 
the  ocean,  where  it  is  lost. 

2.  Drainage. — The  rains  are  constantly  percolating  through  the 
soil  and  carrying  away  dissolved  material.  Since  nitrates  are  soluble, 
large  amounts  are  thus  carried  off  to  the  rivers  which  drain  the  land. 

3.  Decomposition  and  Denitrification. — These  processes,  by 
reducing  organic  nitrogen  to  ammonia  and  by  reducing  nitrates 
to  free  nitrogen,  cause  large  losses.  These  gases  of  course  pass 
from  the  soil  to  join  the  atmosphere.  Such  processes  are  going 
on  wherever  organic  matter  is  found — in  the  river,  the  soil,  the 
manure  heap,  sewage,  and  elsewhere. 

4.  Direct  Chemical  Decomposition. — A  considerable  portion 
of  the  earth's  fixed  nitrogen  is  dissipated  by  direct  chemical  processes. 
Explosions  of  gunpowder  or  other  nitrate  explosives  liberate  free 
nitrogen,  which  goes  at  once  to  join  the  nitrogen  of  the  atmosphere. 

Through  all  these  channels  the  soil  is  being  constantly  deprived 
of  its  nitrogen.  Eventually  it  all  reaches  the  atmosphere,  for 
whether  it  enters  the  ocean  or  dissipates  itself  in  the  streams, 
soil,  or  elsewhere,  by  means  of  decomposition  it  finally  allows  the 
nitrogen  to  pass  of  as  a  free  gas  to  join  nature's  inexhaustible  supply 

90 


THE    LOSS    OF    NITROGEN.  91 

in  the  air.  It  is  quite  necessary  for  the  continuance  of  soil  fertility 
that  this  lost  material  should  be  restored. 

Our  farm  lands  slowly  become  incapable  of  supporting  the 
crops  demanded  of  them.  This  loss  of  fertility  in  worn-out  farms 
is  due,  doubtless,  to  a  number  of  factors,  but  the  loss  of  nitrogen 
is  certainly  the  most  prominent  one.  All  over  the  agricultural 
world  it  has  been  found  necessary  to  replace  this  lost  nitrogen  in 
the  soil.  For  this  purpose  we  have  depended  mostly  upon  commer- 
cial fertilizers,  which  commonly  contain  nitrogen  in  the  form  of 
nitrates.  Of  such  fertilizers  there  is .  a  small  supply  in  the 
world,  chiefly  in  South  America,  and  as  they  are  brought  from 
long  distances  they  are  sold  at  high  prices.  But  the  few  large 
deposits  of  nitrates  in  the  world  are  being  rapidly  exhausted. 
The  high  prices  of  nitrates  are  necessary  and  are  bound  to  increase 
as  the  soil  needs  them  more  and  more  and  as  the  supply  diminishes. 
Clearly  enough,  the  supplying  of  the  lost  nitrogen  will  become 
more  and  more  expensive  as  the  great  nitrogen  stores  are  used  up. 
The  seriousness  of  this  problem  of  a  constant  draining  of  nitrogen 
from  the  soil  has  been  quite  prominent  in  the  minds  of  chemists 
and  agriculturists,  as  they  have  learned  in  the  last  few  years  the 
significance  of  nitrogen  for  agriculture. 

The  continuation  of  agriculture  depends  upon  the  existence 
of  some  means  of  reclaiming  the  nitrogen  from  the  atmosphere 
for  the  use  of  plants.  If  there  is  no  such  means  it  is  evident  that 
the  nitrogen  store  of  the  soil  will  be  used  up  and  vegetation  will 
eventually,  and,  in  highly  cultivated  lands,  speedily  die  of  nitrogen 
starvation.  If,  on  the  other  hand,  there  is  a  possibility  of  reclaiming 
such  lost  nitrogen  there  is  no  need  of  nitrogen  starvation,  since  there 
is  an  absolutely  unlimited  store  of  this. element  in  the  form  of  the  free 
nitrogen  of  the  air.  It  is  quite  evident  that  there  is  some  means 
within  the  reach  of  organic  nature  for  making  usfe  of  this  atmos- 
pheric nitrogen.  Vegetation  has  continued  on  the  earth  for  an 
unknown  number  of  centuries  without  any  apparent  diminution  of 
the  nitrogen  supply.  This  would  not  have  been  possible  unless 
the  soil  could  have  obtained  from  the  air  a  stock  of  nitrogen  to 
replace  that  lost  by  the  processes  already  indicated. 


92  RECLAIMING    LOST    NITROGEN. 

Where  did  the  nitrates  come  from  that  are  now  in  the  soil  ?  Soil  is 
made  of  crumbled  rock  which  did  not  originally  contain  nitrates;  it 
certainly  must  have  obtained  them  from  some  source.  The  various 
bacteria  we  have  been  studying  only  transform  nitrogen  compounds; 
they  do  not  make  a  new  supply.  The  nitrogen  in  the  air  would  be  an 
inexhaustible  source  if  it  were  only  available;  but  the  bacteria  we 
have  considered  have  no  power  of  obtaining  this  nitrogen.  They 
can  transform  nitrogen  compounds,  but  they  cannot  fix  or  gather 
nitrogen  from  the  air.  It  might  naturally  be  supposed  that  ordinary 
plants  could  obtain  nitrogen  from  the  air  as  they  do  CO2.  But  the 
most  careful  testing  has  shown  that  when  such  plants  are  growing 
under  ordinary  conditions  they  cannot  assimilate  any  nitrogen  from 
the  air,  but  must  depend  upon  the  compounds  in  the  soil.  Free 
nitrogen  is  of  no  use  to  them,  only  nitrogen  compounds.  Some 
other  source  of  soil  nitrates  must  be  sought. 

The  Ammonia  Theory. — For  a  time  it  was  held  that  the  am- 
monia in  the  air  was  the  source  from  which  plants  obtained  nitrogen, 
and  that  it  was  carried  into  the  soil  by  the  rains.  When  this  supply 
was  found  to  be  insufficient  to  account  for  soil  nitrates,  it  was 
claimed  that  plants  could  absorb  ammonia  directly  from  the  air 
through  their  leaves.  But  this  theory  failed  to  stand  the  test  of 
experiment,  and  was  finally  abandoned. 

Fixation  of  Nitrates  in  Soil. — It  was  next  shown  that,  under 
proper  conditions,  ordinary  soil  will  increase  its  stock  of  nitrates, 
independently  of  visible  vegetation.  A  lot  of  earth  placed  in  a 
proper  vessel  and  kept  free  from  vegetation  will,  in  time,  be  found  to 
contain  more  nitrates  than  at  the  outset.  Part  of  these  nitrates  may 
be  due  to  the  process  of  nitrification  already  mentioned,  by  which 
the  nitrogen  compounds,  which  were  in  the  soil,  but  not  in  the  form 
of  nitrates,  are  converted  into  nitric  acid  by  the  nitrifying  bacteria. 
But  this  is  not  the  whole  explanation,  because  analysis  of  such  soil 
shows  that  at  the  end  of  several  weeks  there  may  actually  be  a 
larger  amount  of  total  nitrogen  in  the  soil  than  there  was  at  the  start. 
If,  then,  this  total  nitrogen  has  been  increased,  it  must  have  been 
derived  in  some  way  from  the  atmosphere. 


I 

I 


NITROGEN  GATHERING    OR    NITROGEN-FIXING    BACTERIA.         93 

NITROGEN-GATltERING   OR  NITROGEN-FIXING 
BACTERIA. 

It  was  soon  demonstrated  that  nitrogen  fixation  in  the  soil  is  not 
due  to  purely  chemical  processes,  but  rather  to  the  growth  of  micro- 
organisms. That  it  is  due  to  the  action  of  living  organisms  is  proved 
by  the  effect  of  sterilizing  such  soil.  Two  vessels  may  be  filled  with 
similar  soil,  one  of  which  is  sterilized  by  heating,  while  the  other 
serves  as  a  control.  The  former  fails  to  gain  nitrogen,  no  matter 
how  long  it  is  kept  in  contact  with  the  air;  the  latter  slowly  but 
surely  increases  its  store  of  fixed  nitrogen  in  the  form  of  nitrates. 
This  proves  that  some  living  organisms  are  concerned,  and  the  fact 
that  no  visible  plants  are  growing  in  the  soil  shows  that  the  higher 
plants  do  not  produce  the  result.  The  only  conclusion  that  can  be 
drawn,  therefore,  is  that  microorganisms  are  the  agents  for  reclaim- 
ing free  nitrogen  from  the  atmosphere  and  fixing  it  in  the  earth  in 
some  form  of  nitrogen  compounds,  which  eventually  become  nitrates 
and,  thus,  plant  foods. 

Such  facts  plainly  pointed  toward  bacteria  or  allied  organisms  as 
the  real  agents  for  fixing  nitrogen  from  the  air,  and  this  suggestion 
once  made  was  quickly  followed  by  the  isolation  of  the  bacteria  con- 
cerned. It  is  now  a  demonstrated  fact  that  the  power  of  gathering 
atmospheric  nitrogen  and  fixing  it  in  the  soil  belongs  to  bacteria,  and 
during  the  last  fifteen  years  much  study  has  been  devoted  to  the 
microorganisms  concerned.  It  appears  that  there  are  two  general 
types  of  such  nitrogen  fixations  associated  with  different  classes  of 
bacteria:  i.  Nitrogen  fixation  by  bacteria  alone  (non-symbiotic). 
2.  Nitrogen  fixation  by  bacteria  in  connection  with  legumes 
(symbiotic). 

NON-SYMBIOTIC    FIXTURES. 

These  are  soil  bacteria  that  are  able  to  produce  an  increase  of 
nitrogen  in  ordinary  soil  without  the  aid  of  other  organisms.  Of 
these  there  are  two  types,  the  first  acting  anaerobically,  and  the 
second  aerobically.  i.  The  first  one  that  was  found  with  this 
power  was  isolated  from  soil  and  named  Clostridium  pasteurianum 
(Fig.  21).    It  is  an  anaerobic  bacterium,  and  in  culture  media  will  not 


94 


RECLAIMING    LOST   NITROGEN. 


grow  in  the  presence  of  oxygen.  In  the  soil,  however,  it  is  often 
associated  with  a  second  bacterium  that  is  aerobic,  the  latter 
absorbing  the  oxygen  so  that  the  anaerobic  form  can  grow.  Or- 
dinarily the  nitrogen  fixation  in  the  soil  is  due  to  these  two  growing 
together,  but  the  Clostridium  alone  is  able  to  assimilate  nitrogen  if 
kept  in  an  oxygen-free  atmosphere.  In  its  growth  the  bacterium 
consumes  some  of  the  organic  material  in  the  humus,  and  from  this 
source  obtains  the  necessary  energy  for  its  action.  The  organism 
is  widely  distributed,  having  been  isolated  by  several  bacteriologists 
from  dififerent  soils.  Practically  nothing  is  known  as  to  its  activity  in 
soil  under  ordinary  conditions.  2.  In  1901  it  was  proved  that  the 
soil  also  contains  bacteria  of  the  aerobic  type  that  can  fix  nitrogen. 


Fig.  21. — Clos- 
terium  pasteuri- 
anum;  an  anaero- 
bic nitrogen  fixer 
{Winogradski). 


Fig. 22. — 
Azotobacter 
agili s,  an 
aerobic  n  i  - 
trogen  fixer 
(Beyerinck.) 


Fig.  23.-5. 
d aniens,  an 
aerobic  nitro- 
gen fixer  (Loh. 
and  West). 


Two  different  varieties  of  these  were  first  isolated,  and  to  them  was 
given  the  general  name  of  Azotobacter  (Fig.  22).  Several  other 
varieties  have  been  found  later  (Fig.  23).  They  are  considerably 
more  vigorous  than  the  aerobic  type,  and  fix  a  considerably  larger 
amount  of  nitrogen — two  or  three  times  as  much.  In  order  to  de- 
velop efficiently  they  must  be  supplied  with  a  considerable  quantity 
of  organic  food,  and  in  ordinary  soil  the  humus  furnishes  this  food. 
By  the  energy  they  obtain  from  this  source  they  gather  from  the  air 
an  extra  quantity  of  nitrogen.  These  nitrogen  fixers  are  very 
susceptible  to  the  presence  of  the  smallest  amount  of  acid,  and  fail  to 
fix  nitrogen  entirely  if  the  soil  is  even  slightly  acid.  The  use  of  a 
little  lime  to  neutralize  the  acidity  may  thus  frequently  start  an 
active  nitrogen  fixation  in  a  soil  in  which  it  did  not  previously  occur, 
arid  hence  greatly  increase  its  productiveness.  It  has  been  shown 
also  that  this  class  of  nitrogen  fixers,  though  able  to  grow  alone,  will 


SYMBIOTIC    BACTERIA  AND    LEGUMINOUS    PLANTS.  95 

perform  their  functions  best  when  growing  with  certain  other  soil 
organisms.  They  grow  well  with  other  fungi  and  with  some 
algae,  organisms  generally  found  associated  with  them  in  the  soil. 

Other  Nitrogen  Fixers. — It  has  been  claimed  that  other  organ- 
isms are  capable  of  fixing  atmospheric  nitrogen.  Some  species 
of  molds  have  been  placed  in  this  class,  and  certain  species  of  algcsy 
as  well  as  a  considerable  list  of  other  kinds  of  bacteria.  Indeed,  the 
power  of  fixing  nitrogen  has  been  said  by  some  to  be  a  fairly  common 
property  of  bacteria.  Concerning  this  subject  little  is  known  at 
present,  but  it  is  quite  likely  that  the  list  of  nitrogen-fixing  organisms 
will  be  considerably  extended  in  the  next  few  years. 

Nitrogen  Fixation  in  the  Soil. — As  yet  little  is  known  concern- 
ing the  actual  efficiency  of  these  bacteria  in  the  soil,  although  they 
are  certainly  very  active.  Soils  do  gain  nitrogen  and  continue  to 
do  so  for  periods  of  years.  A  long  series  of  tests  has  shown  that 
crops  can  be  removed  from  some  soils  year  after  year  with 
no  diminution  in  the  amount  of  nitrogen  that  may  be  found 
there.  In  these  soils  no  legumes  have  been  grown,  and  hence  it 
would  seem  that  the  supply  of  nitrogen  must  have  come  from  the 
supply  which  the  bacteria  gather  from  the  air.  The  general 
belief  to-day  is  that  this  method  of  fixation  of  nitrogen  is  of  very 
great  significance  in  all  soils,  and  plays  a  much  larger  part  in  the 
maintenance  of  the  soil  nitrates  than  we  formerly  supposed.  It 
is  probable  that  the  bacteria  do  not  fix  the  nitrogen  in  a  form 
immediately  available  to  plants.  They  probably  build  it  into  some 
more  highly  complex  compound,  very  likely  of  a  proteid  nature, 
which  is  incorporated  in  their  own  bodies.  Later  the  processes 
of  decomposition  and  nitrification  act  upon  these  compounds  and 
eventually  convert  them  into  available  nitrates.  Of  this  series 
of  changes,  however,  little  is  yet  known. 

SYMBIOTIC  BACTERIA  AND  LEGUMINOUS. 
PLANTS. 

The  Value  of  Legumes. — It  has  been  known  for  a  long  time 
that  leguminous  plants  in  some  way  enrich  the  soil,  even  the  Romans 
having  commented  upon  the  fact.     The  idea  was  revived  in  the 


96  RECLAIMING    LOST   NITROGEN. 

eighteenth  century  and  has  been  more  or  less  fully  realized  by 
farmers  since  that  time.  To  what  this  soil-enriching  function  is 
due  has  not  been  understood  till  within  the  last  thirty  years.  It 
is  now  known  to  be  due  to  the  fact  that  the  legumes  increase  the 
nitrogen  present.  As  already  noticed,  experimental  evidence 
indicates  that  ordinary  plants  are  unable  to  assimilate  atmospheric 
nitrogen.  Long  series  of  experiments  were  conducted  to  test  the 
matter  and  the  more  rigidly  the  experiments  were  performed, 
the  more  evident  did  it  become  that  such  an  assimilation  does 
not  occur  in  ordinary  green  plants.  It  was,  however,  shown 
in  1883-4  that  this  conclusion  did  not  hold  in  regard  to  the 
great  family  of  legumes.  It  was  demonstrated  very  conclusively 
that  peas  and  beans,  growing  in  a  soil  free  from  nitrogen  and  fed 
upon  food  containing  no  nitrogen,  did,  in  the  course  of  a  few  weeks' 
growth,  increase  the  amount  of  nitrogenous  material  present  in 
the  plant,  and,  inasmuch  as  the  only  possible  source  of  this  nitrogen 
was  the  atmosphere,  the  conclusion  was  unhesitatingly  drawn 
that  peas  can  assimilate  atmospheric  nitrogen.  This  conclusion 
was  contradictory  to  the  belief  accepted  at  the  time,  and  although 
vigorously  disputed,  was  soon  found  to  be  strictly  correct.  Mnay 
of  the  plants  of  the  great  family  of  legumes  certainly  do  have 
the  power,  under  certain  circumstances,  of  fixing  atmospheric  nitro- 
gen and  absorbing  it  into  their  tissues. 

Root  Tubercles. — The  next  step  was  the  observation  that  the 
fixation  of  nitrogen  by  legumes  is  associated  with  the  develop- 
ment upon  the  roots  of  little  nodules  known  as  tubercles  (Fig.  24) . 
These  tubercles  are  little  swellings  on  the  roots,  sometimes  very 
numerous,  and  varying  from  the  size  of  a  pinhead  to  the  size  of  a  pea. 
They  can  easily  be  found  on  nearly  any  legume  growing  luxuriantly 
in  the  soil,  if  the  roots  are  carefully  dug  from  the  soil  in  such 
a  way  as  to  prevent  the  nodules  from  being  destroyed,  and  if  the 
soil  is  carefully  washed  away.  They  were  at  first  regarded  as 
galls  upon  the  roots,  similar  to  those  that  appear  upon  the  leaves 
and  branches  of  trees,  and,  therefore,  were  looked  upon  as  a  type 
of  disease.  It  is,  however,  evident  that  if  they  are  of  the  nature  of 
a  disease,  they  do  the  plants  no  injury,  for  the  plants  developing 


m 

^K  the 


SYMBIOTIC    BACTERIA   AND    LEGUMINOUS    PLANTS. 


97 


no 


these  tubercles  are  as  luxurious  as  those  without  them.  Indeed, 
as  soon  as  the  nitrogen-fixing  power  of  legumes  was  demonstrated, 
it  became  evident  that  the  fixation  of  nitrogen  was  associated  with 
the  formation  of  tubercles.  Only  such  plants  as  develop  tubercles 
are  able  to  increase  the  amount  of  nitrogen  in  their  tissues,  and 
the  amount  of  nitrogen  fixation  is  roughly  proportional  to  the 
development  of  tubercles.  Plants  without  tubercles  show 
increase;  those  with  a  moderate  number,  a 
slight  increase;  and  those  with  abundant 
tubercles  grow  luxuriently  and  show  a  larger 
increase  in  nitrogen. 

These  facts  led  to  experiments  inregard 
to  their  formation  of  tubercles.  The  tuber- 
cles will  not  form  upon  the  roots  of  legumes 
grown  in  sterilized  soil.  Under  these  cir- 
cumstances the  plants  develop  no  tubercles, 
fix  no  nitrogen  and,  unless  fed  with  nitro- 
genous food,  make  very  little  growth,  being 
stunted  and  small.  It  was  next  shown  that 
if  the  legumes  were  sown  in  sterilized  sand, 
without  nitrogenous  food,  and  were  then 
moistened  by  water  which  had  been  stand- 
ing in  contact  with  ordinary  soil,  results  were  quite  different. 
Such  water,  sometimes  called  a  soil  infusion,  is  made  by'simply  soak- 
ing soil  in  water  and  then  filtering  off  the  solid  particles,  using  the 
filtrate  for  watering  the  growing  legumes.  Plants  watered  with  such 
infusions  show  two  interesting  stages  of  growth.  They  sprout 
readily  and  for  a  short  time  grow  vigorously;  then  the  vigorous 
growth  ceases  and  the  plant  seems  to  be  suffering  for  lack  of  food. 
This  has  been  called  the  nitrogen  hunger  stage,  and  represents  a 
period  in  which  the  plant  has  used  up  the  nitrogen  in  the  seed,  and 
consequently  all  that  was  within  reach.  Control  plants,  grown  in 
similar  soil  and  watered  with  pure  water,  never  recover  from 
this  stage,  but  those  that  were  watered  with  the  soil  infusions, 
after  a  few  days  of  such  nitrogen  hunger,  recover,  begin  once 
more  a  vigorous  growth  and  eventually  produce  large-sized  plants 


Fig.  24.— Tubercles  on  the 
roots  of  the  Soy  bean. 


98  RECLAIMING   LOST   NITROGEN. 

with  a  good  yield.  Upon  examining  the  roots  of  the  plants  they  are 
found  to  have  developed  tubercles,  while  the  control  plants,  watered 
with  sterilized  pure  water,  do  not  develop  tubercles.  These  facts 
of  course  indicate  that  in  the  soil  infusion  some  agencies  are 
present  which  stimulate  the  development  of  tubercles  and  the 
consequent  fixation  of  nitrogen,  and  that  the  power  of  absorbing 
atmospheric  nitrogen  enables  the  plant  to  recover  from  the  nitro- 
gen-hunger stage. 

The  Tubercle  Bacteria. — These  facts  naturally  suggest  that 
bacteria,  or  other  microorganisms,  are  the  cause  of  the  tubercles. 
Microscopic  study  of  the  tubercles  shows  a 
somewhat  perplexing  structure.  The  tubercle  is 
the  result  of  the  excessive  growth  of  the  cells  of 
the  root,  but  they  are  filled  with  peculiar  bodies. 
During  the  early  growth  of  the  tubercle,  long, 
thread-like  sacs  appear,  which  force  their  way 
through  the  cells  (Fig.  25).    These  filaments  seem 

.^JJ^:1^'~^a''7^^^  to  be  hollow  tubes  which  contain  smaller  bodies, 
tibethread-Iike  ^  ' 

pouches  in  the  tuber-  somewhat  like  bacteria.  As  the  legume  in- 
represents  ^wo^cells  creases  in  size  these  bacteria-like  bodies  undergo 
with  the  sacs  pene-     a  transformation  in  shape,  growing  larger  and 

tratmgthem(5^e/aw).      ,  ,  .  ,  F  j  8  &         6 

branchmg  somewhat,  so  as  to  form  structures 
like  those  shown  in  Fig.  26.  These  are  called  bacterioids,  and  they 
are  characteristic  of  the  tubercles  of  legumes.  The  next  step  was, 
naturally,  to  isolate  these  bodies  and  study  them  by  bacteriological 
methods.  It  is  easy  to  isolate  from  the  tubercles  bacteria  that  will 
grow  in  culture  media,  and  these  organisms  were  named  B.  radicicola. 
Experiments  with  the  bacteria  thus  isolated  have  been  extensive 
and,  on  the  whole,  satisfactory,  though  occasionally  they  have  been 
conflicting.  It  has  been  proved  many  times  that  tubercles  can  be 
produced  upon  legumes  by  the  cultures  thus  obtained.  Legumes 
have  been  grown  in  sterilized  soil  and  watered  with  bacterial  in- 
fusion from  these  cultures:  the  usual  result  has  been  the  growth  of 
abundant  tubercles  and  the  fixation  of  nitrogen.  Some  striking 
experiments  have  been  made  with  germinating  peas.  Such  peas,  if 
kept  moist  and  warm,  will  grow  for  several  days,  sending  out  their 


I 


SYMBIOTIC    BACTERIA  AND    LEGUMINOUS   PLANTS.  99 

normal  roots  even  without  being  planted  in  the  soil.  By  dipping  the 
tip  of  a  needle  into  cultures  of  the  microorganisms  and  then  pricking 
the  rootlets  of  young  legumes  at  various  points,  the  development  of 
tubercles  will  almost  inevitably  follow  such  slight  wounds.  In 
favorable  experiments  the  tubercles  appear  in  six  days  after  the 
inoculation  and  always  at  the  point  of  inoculation.  These  facts 
proved  that  the  cultures  are  concerned  in  the  development  of  the 
tubercles. 

The  study  of  the  organisms  themselves  and  of  their  relation  to  the 
legume  tubercle  has  proved  somewhat  puzzling.  The  organisms 
isolated  are  ordinary  bacteria,   B.   radicicola,   and   in  laboratory 


Fig.  26. — Showing  the  bacterioids  found  in  root~tubercles. 

culture  media  they  resemble  other  bacteria,  occasionally  producing 
the  peculiar  bacterioid  forms.  Usually  there  is  nothing  in  their 
growth  in  the  laboratory  culture  media  to  suggest  that  they  may  pro- 
duce the  peculiar  bodies  found  in  the  tubercles.  When  such  cultures 
are  inoculated  into  the  roots  of  legumes  the  results  are  not  always 
successful,  sometimes  no  tubercles  following.  But,  as  a  rule,  the 
inoculation  is  followed  by  the  growth  of  the  tubercle,  the  develop- 
ment of  the  curious  tube-like  filaments  growing  among  the  cells,  and 
there  is  the  subsequent  appearance  of  the  bacterioids  in  the  fila- 
ment. The  appearance  of  the  pouch-like  threads  and  the  bacteri- 
oids has  been  a  puzzle  that  has  not  yet  been  wholly  explained. 

It  is  evident  that  the  tubercle  bacteria  must  exist  in  the  soil. 
But  in  spite  of  careful  search  no  bacteria  have  yet  been  isolated 
from  the  soil  which  have  the  power  of  producing  tubercles  when 
inoculated  into  legumes.     This,  together  with  the  irregularity  of 


lOO  RECLAIMING   LOST   NITROGEN. 

results  obtained  in  trying  to  use  the  cultures  of  B.  radicicola,  has  led 
to  suspicions  that  the  actual  bacterium  that  produces  the  tubercle 
may  not  be  the  B.  radicicola  which  has  been  isolated,  but  some 
other,  which  has  escaped  observation  and  which  is  frequently  at- 
tached to  B.  radicicola.  Indeed,  DeRossi  recently  claims  to  have 
found  the  B.  radicicola  associated  with  what  he  thinks  is  a  second 
bacterium  that  has  quite  different  properties.  The  latter  is  much 
more  like  the  bacteria  forms  that  appear  in  the  young  tubercle,  and 
shows  a  tendency  to  form  bacterioids  in  culture  media.  According 
to  DeRossi,  it  always  produces  the  tubercles  when  inoculated  into  the 
root  tissue  of  legumes.  These  bacteria  do  not  grow  well  in  culture 
media,  not  becoming  visible  for  about  two  weeks,  and  have  been 
overlooked  in  previous  experiments  since  they  are  hidden  by  the 
vigorously  growing  B.  radicicola  with  which  they  are  closely  as- 
sociated. DeRossi  thinks  this  a  new  organism  and  the  cause  of  the 
tubercle  rather  than  the  species  ordinarily  accepted  as  the  cause. 
It  is  doubtful  whether  this  is  anything  different  from  B.  radicicola. 
But  whichever  result  is  reached,  it  remains  equally  true  that  the 
tubercles  are  the  result  of  the  action  of  bacteria  that  enter  the  root 
tissues,  and  stimulate  the  root  cells  to  excessive  growth,  although, 
perhaps,  B.  radicicola  is  not  the  real  exciting  cause.  This  conclu- 
sion of  DeRossi,  if  true,  would  in  a  measure  explain  the  irregularity  of 
results  obtained  by  the  use  of  what  were  previously  supposed  to  be 
pure  cultures  of  the  tubercle  organism  (see  page  107). 

The  Production  of  Tubercles  by  the  Bacteria. — Just  how  the 
bacteria  produce  the  tubercle  is  not  known.  Tubercles,  galls,  or 
tumors  are  not  infrequently  produced  in  plants  by  bacteria  and  molds, 
these  constituting  one  of  the  well-known  types  of  plant  diseases. 
Apparently  these  legume  tubercles  are  produced  in  somewhat  the 
same  way,  only  instead  of  injuring  the  plant  they  benefit  it.  It 
appears  too  that  the  plant  offers  some  resistance  to  the  entrance  of 
the  bacteria  into  its  root,  and,  when  well  nourished,  is  able  to  pre- 
vent their  entrance.  When  there  is  plenty  of  nitrogen  food  in  the 
soil,  the  plant  grows  vigorously,  so  that  this  resistance  may  be 
sufficient  to  prevent  the  formation  of  tubercles.  When,  however, 
the  nitrogen  food  is  scanty,  the  plant  is  weaker  and  cannot  resist  the 


ARE  THERE  DIFFERENT  SPECIES  OF  ttlBERCLE  M^J&I^ '?       lOI 

entrance  of  the  bacteria.  The  growing  pea,  when  it  enters  the 
nitrogen-starved  stage  (see  page  97)  becomes  weakened  and  the 
tubercle  organism  readily  penetrates  its  roots.  Thus  it  happens 
that  these  tubercles  form  only  upon  plants  that  grow  in  soils  some- 
what deficient  in  nitrogen,  and  thus  under  exactly  the  conditions 
where  nitrogen  assimilation  is  needed.  Whenever  the  bacteria  do 
succeed  in  entering  the  root  they  stimulate  the  plant  to  excessive 
growth  which  results  in  the  formation  of  tubercles,  and  they  them- 
selves become  transformed  into  the  bacterioids. 

Assimilation  of  Nitrogen. — Just  how  the  nitrogen  is  assimi- 
lated is  also  uncertain.  It  is  possible  that  the  legume  thus  stimu- 
lated can  absorb  the  nitrogen  directly  from  the  air.  A  second 
possibility  is  that  the  bacteria  assimilate  the  nitrogen,  and  that  later 
the  legume  utilizes  this  extra  supply  that  has  been  absorbed.  It  is 
impossible  to  decide  between  these  views,  at  present,  although,  con- 
sidering that  some  bacteria  are  known  to  possess  this  power  of 
nitrogen  fixing  while  green  plants  do  not  have  such  power,  the 
probability  is  in  favor  of  the  latter  view.  However  this  may  be,  it 
is  certain  that  the  legumes  obtain  from  the  growth  of  the  tubercles 
an  extra  supply  of  nitrogen  which  is  derived  from  the  air. 

ARE  THERE  DIFFERENT  SPECIES  OF  TUBERCLE 
BACTERIA? 

It  is  practically  certain  that  nearly  all  soils  contain  bacteria 
capable  of  living  in  symbiosis  with  leguminous  plants.  Nearly 
all  soils,  except  extremely  sandy  soils  that  support  little  or  no 
vegetation,  will  support  leguminous  plants  and  develop  tubercles 
on  their  roots.  One  can  hardly  examine  the  roots  of  legumes  any- 
where without  finding  tubercles,  a  fact  which  shows  that  the  bacteria 
in  question  are  very  widely  distributed  in  nature.  But  are  the 
bacteria  all  of  the  same  species?  A  very  large  number  of  species 
of  legumes  with  their  tubercles  can  grow  in  most  if  not  all  soils. 
Are  the  bacteria  that  form  tubercles  upon  the  clover  the  same  as 
those  that  form  them  upon  the  pea,  or  is  there  a  different  species 
of  bacteria  for  the  different  species  of  legume  ?     It  would  not  seem 


I02    .    \  :  ^^  ^  >r  '^  ^^  Ki:iPMI»^^NG    LOST   NITROGEN. 

probable  that  there  could  be  in  the  soil  a  different  variety  of  bacteria 
for  every  variety  of  legume,  but  rather  that  one  kind  of  bacteria  can 
grow  in  many  legumes.  But  the  facts  are  not  quite  so  simple  as  this. 
Not  all  species  of  legumes  are  capable  of  developing  root  tubercles 
equally  well  in  all  soils.  Some  soils  will  luxuriantly  support  certain 
species  of  beans,  peas,  or  clovers,  producing  a  large  crop,  developing 
quantities  of  tubercles  and  fixing  an  abundance  of  nitrogen,  while 
the  same  soil  will  not  support  other  species  of  legumes  with  equal 
readiness.  For  example,  the  soil  of  Connecticut  is  not  adapted 
to  the  legume  called  the  soy  bean.  When  this  bean  is  planted  in  the 
ordinary  Connecticut  soil  it  does  not  flourish,  but  yields  a  small 
crop  unless  heavily  fertilized,  and  does  not  produce  tubercles. 
This  species  does,  however,  grow  readily  in  Massachusetts.  Some 
years  ago  the  experiment  was  tried  of  importing  Massachusetts 
soil,  upon  which  this  plant  had  produced  abundant  tubercles, 
and  mixing  it  with  the  Connecticut  soil,  subsequently  planting  the 
soy  bean.  The  result  was  an  excellent  growth  of  the  soy  bean 
and  the  development  of  tubercles.  Afterward  these  particular 
plots  of  land  were  capable  of  producing  large  luxuriant  crops  of 
the  soy  bean,  with  abundant  root  tubercles  and  a  large  fixation 
of  atmospheric  nitrogen.  Evidently  Connecticut  soil  does  not 
contain  the  bacteria  adapted  for  producing  the  tubercles  in  the  soy 
bean,  although  those  which  produce  tubercles  on  the  pea  and  the 
clover  are  abundant  enough. 

Similar  experiments  have  been  repeated  elsewhere  until  it  has 
become  evident  that  the  root  tubercle  bacteria  are  not  all  alike. 
Varieties  adapted  to  one  species  of  legume  may  be  unable  to  produce 
tubercles  upon  a  second  species;  in  some  cases  one  type  of  bacteria 
may  be  able  to  grow  in  the  roots  of  several  allied  legumes  but  not 
in  others.  For  example,  the  tubercle  organism  of  sweet  clover 
will  do  well  with  alfalfa.  All  of  these  facts  have  suggested  that 
there  are  different  types  of  leguminous  bacteria,  each  adapted  to 
different  species  of  legumes. 

To  what  extent  this  conclusion  is  true  it  is  by  no  means  easy  to 
determine.  It  is  certainly  true  that  some  varieties  of  legume^will 
grow  in  soils  with  an  abundant  production  of  tubercles,   while 


ARE  THERE  DIFFERENT  SPECIES  OF  TUBERCLE  BACTERIA?       IO3 

Other  varieties  of  closely  related  legumes  are  unable  to  produce 
an  abundant  crop  of  tubercles  in  the  same  soil.  This  is  evidently 
due  to  the  lack  of  microorganisms  especially  appropriate  to  the 
legume  in  question,  since  inoculation  with  proper  soil  infusion  pro- 
duces tubercles  at  once.  But  just  what  this  means  is  not  so  evident. 
It  certainly  means  that  different  legumes  demand  different  varieties 
of  tubercle  bacteria.  Whether  these  different  varieties  are  distinct 
species  is,  of  course,  a  fruitless  question  inasmuch  as  we  do  not 
know  what  we  mean  by  a  species  among  bacteria.  But  it  is  of 
importance  to  know  whether  these  types  are  quite  distinct  or 
whether  they  are  simply  physiological  varieties  of  the  same  general 
species.  If  the  former  is  true  we  should  expect  them  to  remain 
distinct,  but  if  the  latter  is  true  we  might  expect  the  soil  bacteria 
to  be  capable  of  adaptation,  by  cultivation,  to  different  legumes. 
On  the  whole,  the  evidence  is  decidedly  in  favor  of  the  latter  view 
and  indicates  that  the  different  tubercle  bacteria  are  probably  all 
one  general  species,  but  that  under  different  conditions  they  assume 
slightly  different  physiological  relations.  They  can  accommodate 
themselves  to  growth  in  one  or  another  legume,  and  having  become 
especially  adapted  to  one  species,  they  do  not  so  readily  develop  in 
the  root  of  a  second  species,  but,  allowed  to  develop  in  the  soil  in 
which  the  latter  plants  are  growing,  they  adapt  themselves  in  time 
to  the  new  plant.  In  other  words,  experiments  indicate  that  there 
is  probably  only  one  species  of  tubercle  bacteria,  and  that  this 
species  assumes  different  physiological  characters  under  the  influence 
of  the  different  conditions  in  which  it  grows.  It  may  adapt  itself 
especially  for  growth  in  one  leguminous  plant  and  consequently 
lose  its  ability  to  develop  well  in  others;  but  if  a  new  legume  is 
planted  in  the  same  soil,  a  slow  change  of  physiological  characters 
takes  place,  and  the  soil  organism  becomes  in  time  adapted  to  the 
new  leguminous  plant.  This  conclusion  is  clearly  in  complete 
harmony  with  the  fact  that  the  soil  may  at  any  time  contain  the 
organisms  which  will  support  one  species  of  legume  luxuriantly, 
while  another  species  will  have  only  a  scanty  growth.  The  matter 
of  practical  importance  is  that  a  soil  may  support  one  species  of 
legume    luxuriantly,    with    abundant    tubercle    production,    while 


I04  RECLAIMING    LOST   NITROGEN. 

a  second  species  will  not  flourish  upon  it  because  of  lack  of  tubercle 
bacteria  properly  adapted  to  the  second  species. 

THE  UTILIZATION  OF  THE  NITROGEN-FIXING 
POWERS  OF  LEGUMES. 

Although  there  are  still  unsettled  questions  concerning  the 
nature  of  the  tubercles,  the  power  possessed  by  legumes  of  fixing 
nitrogen  through  their  aid  is  of  the  utmost  importance.  The 
legumes  are  the  most  practical  means  within  reach  for  restoring  to 
the  soil  the  nitrogen  dissipated  into  the  air,  and  it  becomes  a 
matter  of  great  significance  to  agriculture  to  determine  the  best 
practical  method  of  making  use  of  this  power.  It  would  seem  that 
we  have  here  the  factor  needed  for  making  possible  a  cultivation  of 
the  soil  without  exhausting  its  nitrogen.  Virgin  soil  has  all  its 
factors  of  nitrogen  loss  and  gain  nearly  balanced;  cultivated  soils 
have  a  balance  on  the  debit  side.  If  we  can  discover  a  practical 
method  of  applying  these  factors  of  nitrogen  assimilation,  one  of  the 
great  agricultural  problems  will  be  solved.  Up  to  the  present  time 
the  matter  has  not  been  brought  to  a  condition  where  we  can  feel 
that  we  know  how  to  handle  these  nitrogen-fixing  forces  to  the 
greatest  advantage;  but  so  much  has  been  learned  that  already 
our  agriculturists  are  making  use  of  the  knowledge  to  a  great  extent, 
and  we  may  fairly  expect  that  the  next  few  years  will  see  these 
forces  more  thoroughly  under  the  control  of  the  farmer  than  to-day. 

In  making  use  of  this  means  of  gaining  nitrogen  the  following 
facts  must  be  considered. 

I.  Selection  of  a  Proper  Legume. — The  question  of  the  proper 
legume  to  grow  in  any  soil,  for  the  purpose  of  fixing  its  soil  nitrogen, 
is  one  that  must  be  determined  largely  by  experiment.  In  all  cases 
it  should  be  the  legume  that  grows  most  luxuriantly  upon  soils  not 
particularly  well  fertilized,  and  which,  at  the  same  time,  produces 
the  most  abundant  crop  of  tubercles  upon  its  roots.  These  factors 
will  depend  upon  climate,  the  chemical  nature  of  the  soil  and  the 
variety  of  soil  bacteria.  In  selecting  the  legume  the  individual 
must  take  into  consideration  all  the  facts  within  his  reach.     Some 


UTILIZATION    OF    NITROGEN-FIXING    POWERS    OF    LEGUMES.       IO5 

species  grow  better  in  some  climates  than  others,  and  certain  soils 
seem  to  be,  for  some  reason,  better  adapted  for  particular  species, 
quite  independent  of  the  question  of  the  presence  of  the  proper  soil 
bacteria.  By  the  proper  consideration  of  the  facts  of  his  experience 
the  farmer  can,  without  much  difficulty,  determine  what  species  of 
legume  grows  best  in  his  soil.  The  most  vigorously  growing 
legume  is  the  best.  In  clay  soils  red  and  yellow  clover,  lupin,  sera- 
dilla,  horse  beans,  and  vetches  are  successfully  grown.  Which  of  the 
varieties  is  to  be  selected  must  be  determined  by  the  conditions  of 
the  soil  and  the  needs  of  the  farmer  for  the  particular  crop  which  he 
raises.  The  essential  feature  must  be  that  the  species  selected 
should  be  one  that  will  grow  well  in  the  soil  in  question,  otherwise 
the  advantage  of  the  nitrogen  fixation  will  not  be  obtained. 

2.  Insuring  Presence  of  Proper  Bacteria. — In  order  that  the  soil 
may  increase  its  nitrogen  store  it  is  evidently  necessary  for  tubercles 
to  develop  in  large  numbers  on  the  roots  of  the  legumes.  For  this 
purpose,  of  course,  it  is  necessary  that  the  proper  variety  of  bacteria 
shall  be  present  in  the  soil,  otherwise  no  tubercles  will  be  formed,  or 
the  tubercles  formed  will  be  few  and  small.  To  insure  this  result 
may  sometimes  require  a  little  experimenting  and  observation. 
Some  species  of  legume  find  in  a  certain  soil  the  tubercle  organism 
adapted  to  them,  while  other  species  of  legume  may  not  find  the 
proper  organisms  in  the  same  soil.  The  soy  bean  is  a  most  excellent 
crop  for  nitrogen  gathering  since  it  is  an  extremely  luxurious  grow- 
ing legume,  producing  abundant  tubercles  and  a  large  fixation  of 
nitrogen  when  supplied  with  the  organisms  which  produce  tubercles. 
But  in  order  to  make  use  of  this  crop  it  may  be  necessary  to  import 
the  proper  bacteria  from  other  soils.  On  the  other  hand,  there  are 
some  species  of  legumes,  like  most  kinds  of  peas,  which  are  capable 
of  growing  in  most  soils  and  producing  an  abundance  of  tubercles. 

Further,  a  legume,  which,  during  the  first  season  produces  only  a 
small  number  of  tubercles,  may  succeed  better  the  second  year  than 
the  first  and  may  fix  more  nitrogen.  The  growth  of  the  crop  in  the 
soil  during  the  first  year  apparently  either  increases  the  number  of 
soil  organisms  appropriate  to  this  particular  legume  or  produces 
such  changes  in  the  physiological  character  of  the  bacteria  present 


I06  RECLAIMING    LOST    NITROGEN. 

that  they  are  better  adapted  to  the  legume.  In  either  case,  the 
second  season  will  show  a  more  luxuriant  growth  and  a  more  suc- 
cessful nitrogen  fixation. 

Soil  Inoculations. — Experience  has  shown  that  it  is  not  always 
possible  to  get  a  good  growth  of  the  desired  legume,  because  of  the 
failure  to  obtain  a  proper  quantity  of  tubercles.  That  this  is  due 
to  the  lack  of  the  right  variety  of  bacteria  in  the  soil  seems  certain, 
and  has  led  to  the  practice  of  inoculating  the  soil.  The  first  method 
of  doing  this  is  to  obtain  soil  from  some  locality  where  the  legume  is 
known  to  produce  a  goodly  numbers  of  tubercles  and  then  either  to 


Fig.  27. — Two  snap-bean  plants,  growing  in  coal  ashes,  one  with 
and  one  without  inoculation  {Ferguson). 


mix  this  soil  with  that  of  the  field  to  be  planted,  or  to  make  a  soil  infu- 
sion to  be  used  for  soaking  the  seeds  or  for  watering  the  young  plants. 
The  results  of  this  procedure  are,  in  the  main,  satisfactory,  for 
generally  the  production  of  tubercles  is  thus  stimulated,  and  much 
increased  crops  produced  (Fig.  27).  In  many  instances  of  this 
kind  it  has  been  found  possible  to  cultivate  a  legume  in  soils  in  which 
it  would  not  previously  grow,  by  simply  inoculating  the  new  soil 
with  soil  where  the  legume  has  previously  grown.  Alfalfa,  for  ex- 
ample, has  been  successfully  started  by  this  means  in  many 
places  in  the  eastern  part  of  this  country  where  it  would  not  pre- 


UTILIZATION    OF    NITROGEN-FIXING    POWERS    OF    LEGUMES.       I07 

viously  grow.  There  seems  no  doubt  that  the  phenomenon  is 
simply  one  of  inoculating  the  soil  with  the  proper  bacteria. 

But  soil  inoculations  with  legume  earth  are  troublesome.  Soil  is 
bulky  and  a  considerable  quantity  is  needed.  To  obtain  a  sufficient 
amount  involves  expensive  freight  charges  and  the  carting  of  heavy 
loads.  Soil  inoculations  may  also  distribute  plant  diseases  and 
troublesome  weeds.  If  tubercles  are  produced  by  bacteria  it  ought 
to  be  possible  to  obtain  the  results  by  inoculating  with  pure  cultures 
of  bacteria.  It  should  be  possible  to  cultivate  the  bacteria  in  a 
laboratory  and  then  to  distribute  to  the  farmers  the  cultures  of  the 
organisms.  If  this  could  be  done  it  would  be  a  far  simpler  matter 
than  the  use  of  soil  itself.  The  first  attempt  to  furnish  such  a  culture 
resulted  in  an  article  called  Nitragin,  which  was  brought  out  in 
Germany.  This  product  was  eagerly  tried  by  experimenters  and 
practical  farmers;  but,  although  in  some  cases  it  seemed  to  give 
favorable. results,  the  success  attending  its  use  was  so  uncertain  that 
it  fell  into  disrepute.  Later,  various  improvements  were  introduced 
into  the  methods  of  making  and  distributing  the  cultures,  and  a 
new  product,  called  New  Nitragin,  has  been  put  forward  which 
gives  somewhat  better  results.  This  has  been  tried  quite  exten- 
sively in  the  last  three  years  and  the  results  have  been  much  more 
positive  than  in  the  earlier  attempts.  Meantime  other  attempts 
toward  the  same  end  were  made  in  this  country.  In  the  labora- 
tories of  the  Department  of  Agriculture  extensive  experiments  were 
carried  out,  seeming  to  show  the  possibility  of  increasing  the  nitro- 
gen-fixing powers  of  these  bacteria  by  cultivating  them  in  solutions 
that  are  poor  in  nitrogen.  After  continued  experiments  in  this  line, 
cultures  of  the  tubercle-producing  organisms  were  sent  out  for  test- 
ing from  the  department.  These  too  proved  unsatisfactory.  They 
were  first  sent  out  upon  absorbent  cotton,  but  the  bacteria  did  not 
live  long  on  the  cotton  fibers  and  the  farmers  were  likely  to  receive 
cotton  containing  only  dead  bacteria.  Then  they  were  sent  out  in  a 
liquid  or  upon  agar  but  their  use  has  never  been  very  great. 

It  thus  appears  that  the  use  of  pure  cultures  of  tubercle  organisms 
for  soil  inoculation  has  hitherto  not  been  very  successful.  But  this 
does  not  by  any  means  indicate  that  the  methods  will  not  soon  be  so 


Io8  RECLAIMING    LOST   NITROGEN. 

improved  that  they  will  be  practical.  The  plan  is  logically  a  proper 
one,  and  if  it  be  found  possible  to  develop  the  tubercle  bacteria  in 
sufl&cient  quantity  and  to  distribute  them  in  a  living  condition, 
these  soil  inoculations  may  become  of  great  value  to  the  agricultural 
industry. 

The  reasons  for  failure  thus  far  are  varied.  The  difficulties  of 
keeping  the  cultures  pure,  of  distributing  them  to  the  farmers  in 
a  still  vigorous  condition,  and  of  finding  some  device  by  which  the 
farmers  can  successfully  inoculate  legumes  with  cultures  have 
been  regarded  as  the  primary  obstacles.  Moreover,  some  soils 
are  already  stocked  with  proper  tubercle  bacteria  so  that  the  addi- 
tion of  more  would  be  superfluous.  If,  however,  the  claims  of 
DeRossi  are  correct  and  the  tubercles  are  caused  not  by  the 
B.  radicicola,  but  by  another  much  more  slowly  growing  organism, 
the  irregularities  in  these  results  are  readily  explained,  and  it  will 
be  necessary  to  proceed  along  a  different  line  in  developing  cultures 
of  the  organism  that  really  produces  the  tubercles.  At  the  present 
time,  therefore,  the  pure  cultures  of  the  tubercle  organisijis  that 
have  been  put  on  the  market  for  soil  inoculation  are  not  reliable, 
although  we  may  confidently  expect  that  the  methods  will  be  so 
improved  in  the  future  as  to  make  them  of  practical  value.  Mean- 
time, soil  inoculations  continue  to  be  made  with  legume  earth 
from  lands  where  the  desired  legumes  are  growing  vigorously;  and 
this  method  of  soil  inoculation  has  proved  of  much  practical  value 
in  developing  a  vigorous  growth  of  legumes  and  a  consequent  in- 
creased fixation  of  atmospheric  nitrogen. 

3.  Utilization  of  the  Nitrogen. — The  next  problem  is  how 
such  a  store  of  nitrogen,  fixed  in  the  soil,  may  best  be  utilized  for 
the  benefit  of  the  next  crop.  There  are  two  methods  by  which 
this  nitrogen  may  be  made  available  for  crops  subsequently 
growing  in  the  same  soil.  The  first,  which  is  commonly  called 
green  manuring,  consists  in  allowing  the  legume  to  grow  vigor- 
ously for  a  time,  and  then  in  plowing  the  whole  crop  into  the  soil,  with 
the  expectation  that  the  nitrogen  stored  up  in  the  plants  will  be 
available  in  the  soil  for  the  next  crop.  The  method  by  which  the 
nitrogen  becomes  available  is  based  upon  the  facts  already  noticed. 


UTILIZATION   OF   NITROGEN-FIXING   POWERS    OF    LEGUMES.       IO9 

When  these  crops  are  thus  plowed  into  the  soil  they  are  brought 
at  once  within  reach  of  the  soil  bacteria. 

The  bacteria  seize  hold  of  the  proteid  products  in  the  plants,  as 
well  as  the  cellulose  and  other  organic  substances,  and  cause  their 
rapid  decomposition.  After  this  process  is  finished  the  nitrifying 
bacteria  in  the  soil  oxidize  the  ammonia  left  after  the  decomposition 
ceases  and  convert  it  into  nitrate.  Thus,  after  a  few  weeks,  a  con- 
siderable portion  of  the  nitrogen  material  which  was  fixed  in  the 
legume  has  been  converted  into  nitrate,  available  for  plant  life. 
These  remain  in  the  soil  and  may  be  used  by  the  next  crop  of  plants 
sown  on  the  same  field,  thus  increasing  its  yield  by  means  of  the 
nitrogen  which  has  been  fixed  by  the  legume  and  the  bacteria 
together,  and  has  been  converted  into  an  available  form  by  the  soil 
bacteria. 

A  second  method  of  utilizing  the  nitrogen  is  by  converting  it 
into  manure.  The  crop  of  legumes  is  reaped  and  fed  to  animals, 
the  roots  and  stubble  only  being  plowed  into  the  soil.  The  portion 
fed  to  the  animals  is  later  returned  to  the  soil  as  manure.  Part 
of  the  nitrogenous  material  is  thus  metabolized  by  the  animal  body 
to  urea,  and  part  passes  into  the  feces  unassimilated,  while  part 
remains  in  the  roots  and  soil.  But  it  is  all  eventually  decomposed 
by  the  putrefying  bacteria,  and  goes  through  the  same  series  of  meta- 
morphoses which  we  have  already  described  in  sufficient  detail.  The 
result  is  that,  in  the  end,  most  of  it  is  returned  to  the  soil  in  a  form 
available  for  plant  life.  This  method  of  utilizing  the  nitrogen  is 
certainly  the  best  economy,  since  it  has  a  double  advantage:  The 
nitrogen  is  used  twice,  once  as  a  food  for  the  stock  and  a  second 
time  as  a  food  for  the  crops  in  the  form  of  manure. 

It  is  of  course  manifest  that  under  either  of  these  methods  of 
treatment  not  all  of  the  nitrogen  fixed  by  the  legume  and  the 
bacteria  is  rendered  available  for  the  next  series  of  crops.  At 
the  very  best,  part  of  it  will  be  lost  to  the  soil  by  the  process  of 
putrefaction  which  liberates  free  ammonia,  and  by  denitrification 
which  liberates  free  nitrogen.  It  is  impossible,  by  any  means  now 
at  our  disposal,  to  prevent  this  loss,  and  thus  a  portion  of  the  fixed 
nitrogen  is,  even  with  the  best  treatment,  dissipated  again  into 


no  RECLAIMING    LOST   NITROGEN. 

the  air.  But  by  proper  treatment  this  loss  can  be  reduced  to 
a  minimum  and  there  may  always  be  a  surplus  of  gain.  Even 
taking  into  account  all  the  nitrogen  loss  that  comes  from  these 
processes  the  use  of  a  leguminous  crop  upon  a  soil  poor  in  nitrogen 
furnishes  to  that  soil  for  the  next  crop  a  store  of  nitrogen  considerably 
in  excess  of  that  which  it  possessed  before. 


CHAPTER  VIII. 
BACTERIA  AND  SOIL  MINERALS. 

MINERALS  NECESSARY  FOR  PLANTS. 

Although  the  different  minerals  in  the  soil  are  needed  by  plants 
in  smaller  quantities  than  the  nitrogenous  foods,  still  they  are  quite 
as  necessary,  and  vegetation  cannot  be  supported  without  them. 
They  come  primarily  from  the  rocks  that  form  the  earth's  crust. 
In  these  rocks  there  is  practically  an  unlimited  supply  of  the  neces- 
sary minerals,  but  they  must  be  rendered  available  as  plant  food. 
Most  of  these  rocks  contain  their  minerals  in  an  insoluble  condition 
and,  in  order  to  be  absorbed  by  vegetation,  they  must  be  dissolved 
in  the  soil  waters.  Although  this  subject  has  not  been  studied  so 
thoroughly  as  the  transformation  of  nitrogen,  still  it  is  known  that 
chiefly  through  the  agencies  of  the  soil  microorganisms  the  minerals 
are  brought  into  solution. 

LIME  AND  MAGNESIA. 

These  two  minerals  may  be  considered  together  since  they  are 
closely  allied  and  their  relations  are  the  same.  The  importance  of 
lime  to  soil  has  long  been  recognized  and  our  previous  study  has 
shown  one  of  its  most  important  uses.  We  have  learned  how 
necessary  is  the  activity  of  the  soil  bacteria  in  the  transformation  of 
plant  foods,  and  how,  as  a  rule,  bacteria  cannot  grow  in  the  presence 
of  the  slightest  acid  reaction.  But  general  processes  of  decomposi- 
tion are  constantly  giving  rise  to  acids,  so  that  the  soils  tend  to 
become  more  and  more  acid.  As  the  acidity  increases  the  bacterial 
action  declines  and  fertility  correspondingly  diminishes.  The 
addition  of  lime  to  such  soils  is  necessary,  therefore,  to  reduce  the 
acidity.  Other  needs  there  may  be  for  lime,  but  the  primary  one  is 
to  keep  the  bacterial  activities  in  the  soil  at  a  high  state  of  activity. 

Ill 


112  BACTERIA  AND    SOIL   MINERALS. 

But  there  are  also  constant  losses  of  lime  from  the  soil.  A  small 
quantity  is  carried  away  by  the  crops  taken  from  the  land,  but  a  far 
larger  quantity  is  lost  to  the  soil  by  drainage.  The  soluble  lime 
salts  are  dissolved  by  the  soil  waters  and  pass  off  with  the  drainage. 
Very  large  amounts  are  thus  removed  so  that  a  more  or  less  frequent 
liming  is  necessary  to  maintain  in  the  soil  a  quantity  sufficient  to 
keep  the  proper  condition  for  bacterial  action.  Different  soils  show 
wide  differences  in  the  amount  of  lime  needed.  Soils  containing 
limestone  rock  have  an  abundant  natural  supply,  while  soils  without 
limestone  need  to  be  furnished  with  it  in  varying  amounts.  The 
lime  thus  drained  away  is  a  permanent  loss,  for  it  finds  its  way  into 
the  ocean  whence  it  is  not  easily  returned  to  the  soil.  But  this 
loss  is  not  serious,  since  limestone  rocks  are  practically  unlimited 
and  there  need  be  no  lack  in  the  supply  of  available  lime.  Lime  is 
rendered  available  chiefly,  if  not  wholly,  through  the  action  of  bac- 
teria. Limestone  consists  mainly  of  carbonate  of  lime  which  is 
only  very  slightly  soluble  in  water,  and  cannot  be  utilized  directly, 
for  this  reason.  But  water  containing  carbonic  dioxid  in  solution 
readily  dissolves  the  carbonate  of  lime.  We  have  seen  that  by  the 
constant  decomposition  processes  going  on  in  the  soil,  carbon 
dioxid  gas  is  being  set  free  from  the  decomposing  organic  com- 
pounds, such  as  proteids,  sugar,  cellulose,  etc.  This  gas  is  taken  up 
by  the  water,  which  is  then  able  to  dissolve  the  limestone.  The 
greater  the  extent  of  the  bacterial  action,  the  greater  will  be  the 
amount  of  carbon  dioxid  eliminated,  and  the  amount  of  lime 
brought  into  solution;  the  more  effectually  also  will  the  soil  be  main- 
tained in  proper  condition  for  bacterial  growth.  Hence,  as  the 
amount  of  lime  in  the  soil  increases,  the  bacterial  action  will  become 
greater,  more  lime  will  be  dissolved,  and  consequently  more  will  be 
lost  by  drainage. 

In  this  way  the  limestones  on  the  earth's  crust  are  being  dis- 
solved and  carried  away.  The  extent  to  which  this  is  possible  is 
indicated  by  the  huge  limestone  caves  whose  great  spaces  show  how 
the  limestone  has  been  dissolved  by  waters  which  held  carbon  di- 
oxid in  solution.  All  such  dissolved  lime  finds  its  way  to  the  ocean 
where  it  supplies  marine  animals  and  plants  with  the  lime  for  their 


PHOSPHORUS. 


113 


shells,  and  where  it  is  also  laid  down  in  the  deposits  of  lime  material 
that  may,  in  later  ages,  form  new  limestone  rocks.  But,  aside  from 
this  future  possibility,  the  bacterial  agencies  of  the  earth's  surface 
are  constantly  dissolving  the  limestones  and  adding  them  to  the 
soil  to  be  subsequently  carried  away  by  drainage. 

In  recent  years  calcium  cyanid  has  become  much  used  as  a 
fertilizer.  It  furnishes  lime  and  results  in  distinct  nitrogen  gains 
to  the  soil.  In  the  utilization  of  this  material  bacteria  are  necessary 
to  convert  it  into  ammonium  salts  before  it  can  be  assimilated  by 
plants. 

PHOSPHORUS. 

Vegetation  needs  only  very  small  amounts  of  phosphorus,  but 
these  small  amounts  are  requisite  to  the  production  of  good  crops,  as 
has  been  many  times  appreciated  by  the  farmer  who  finds  decidedly 
increased  crops  following  the  application  of  phosphate  fertilizers  to 
the  soil.  There  are  many  substances  containing  phosphorus  which 
may  be  used  to  supply  the  amount  needed  by  the  soil.  They  are: 
I.  Mineral  compounds,  of  which  the  chief  are  ground  phosphate  rock 
(floats),  superphosphates  and  a  by-product  of  steel  manufacture 
called  Thomas  slag.  2.  Organic  compounds.  A  considerable  quan- 
tity of  phosphorus  is  contained  in  the  humus,  likewise  in  bone,  which 
is  used  as  a  fertilizer  chiefly  for  its  phosphorus.  The  solid  part  of 
barnyard  manure  contains  phosphorus,  and  a  variety  of  other 
sources,  are  also  utilized — ground  fish,  tankage,  castor  pomace,  and 
the  like.  The  phosphorus  in  some  of  these  substances  is  readily 
soluble  in  water,  and  this  must  always  be  the  case  before  it  can  be 
utilized  by  plants. 

Apparently  the  solution  of  the  phosphates  is  dependent  upon 
bacterial  action.  It  is  easy  to  understand  how  the  phosphorus  from 
organic  sources  is  rendered  available  through  the  agency  of  the  soil 
organisms.  As  these  bacteria  decompose  the  various  organic  prod- 
ucts in  the  soil,  the  phosphorus  contained  in  them  is  set  free  from 
its  combinations.  Bone,  for  example,  is  vigorously  attacked  by  the 
bacteria,  and  is  in  time  completely  disintegrated,  the  phosphorus 
being,  of  course,  freed  from  its  relations.     The  entire  series  of 


114  BACTERIA  AND    SOIL   MINERALS. 

changes  through  which  it  passes  is  not  yet  known.  Part  of  the 
phosphorus  finally  assumes  an  insoluble  condition,  but  a  part  is  dis- 
solved in  the  soil  water  and  becomes  available  as  plant  food.  This 
solvent  action  of  the  bacteria  is  attributable  to  the  acids  that  are 
produced.  As  we  have  already  seen,  the  decomposition  of  organic 
products  always  gives  rise  to  certain  organic  acids  and  these  are 
capable  of  dissolving  phosphorous  compounds  that  would  be  in- 
soluble in  water  alone.  The  solvent  action  resulting  from  bacterial 
decomposition  is  not  wholly  the  result  of  the  acids,  for  by  some 
means  yet  unknown  the  phosphorous  compounds  may  be  dis- 
solved even  when  no  acid  is  produced.  They  are  not,  however, 
dissolved  in  sterile  soil;  therefore  the  availability  of  the  phosphorus 
is  due  to  bacterial  activities.  Such  a  formation  of  soluble  phos- 
phorus from  decaying  organic  compounds  is  going  on  constantly  in 
the  humus,  and  in  soils  rich  in  humus  the  process  furnishes  phos- 
phorus in  sufficient  quantity  for  vegetation. 

Sometimes,  however,  more  phosphorus  is  needed,  and  it  may  be 
supplied  by  minerals.  The  rock  phosphates  are  rendered  available 
in  much  the  same  manner  as  the  organic  phosphates.  The  phos- 
phorous compounds  in  the  rock  are  very  slightly,  if  at  all,  soluble  in 
water.  In  ordinary  soil  small,  but  sufficient  quantities  are  dis- 
solved through  the  agency  of  the  soil  bacteria.  Hence  also  the 
acids  produced  by  decomposition  are  important  agents  in  dissolving 
the  rock  which,  though  not  soluble  in  water,  is  soluble  in  acids.  It 
is  a  well-known  fact  that  these  phosphates  are  made  more  available 
as  a  fertilizer  by  being  composted  for  a  time  in  manure,  a  fact 
clearly  explained  by  the  solvent  action  of  the  acids  produced  by 
decomposition,  as  well  as  by  other  functions  of  the  bacteria  not  yet 
understood.  They  are  also  made  more  effective  when  plowed  into 
the  ground  with  the  plants  used  for  green-manuring,  this  condition 
giving  rise  to  rapid  bacterial  action,  resulting  in  a  decomposition 
which  aids  in  rendering  the  phosphorous  compounds  available. 
Thomas  slag  is  also  dissolved  by  similar  activities.  In  short,  while 
bacteria  do  not  furnish  phosphorus,  they  are  the  active  agents  in 
rendering  available  the  phosphorus  from  both  organic  and  mineral 
sources. 


POTASH — SULPHUR.  H^ 

POTASH. 

The  relation  of  potash  in  the  soil  is  almost  exactly  the  same  as 
that  of  phosphorus.  It  comes  primarily  from  the  rocks  where  it 
exists  largely  in  the  form  of  silicate  of  potassium.  This  is  an  insoluble 
salt,  and  soils  may  contain  it  in  large  quantity  and  still  suffer  from 
lack  of  available  potash.  It  is  rendered  available  in  very  much  the 
same  way  as  in  the  case  of  phosphorus,  largely  through  the  action 
of  the  decomposition  produced  by  the  soil  bacteria. 

SULPHUR. 

Sulphur  is  one  of  the  ingredients  of  protein,  and,  therefore, 
is  necessary  to  plant  life.  Ordinary  plants  obtain  it  only  in  the  form 
of  sulphate,  which  they  absorb  from  the  soil.  But  microorganisms 
are  concerned  in  the  transformations  by  which  the  soil  is  properly 
stocked  with  the  sulphates.  The  transformations  show  at  least  two 
different  steps: 

I.  Sulphur  is  set  free  from  its  combinations.  2.  Sulphur  is 
reconibined  into  sulphuric  acid  that  unites  with  mineral  matter  to 
form  sulphates. 

Liberation  of  Sulphur  as  H^S. — All  proteid  matter  contains 
sulphur,  and  when  its  decomposition  takes  place  through  the  agency 
of  bacteria  the  sulphur  is  liberated  in  the  form  of  hydrogen  bi- 
sulphid  (HjS)  which  vile-smelling  gas  may  usually  be  detected 
around  decomposing  proteid.  This  same  gas  is  liberated  from  the 
decomposition  of  sulphate  of  lime  that  is  carried  in  drainage  waters 
to  the  ocean.  Several  kinds  of  bacteria  have  been  found  capable 
of  liberating  HjS  from  such  deposits.  In  certain  parts  of  the  world 
large  deposits  of  such  sulphites  (gypsum)  have  accumulated  and 
are  constantly  acted  on  by  bacteria  which  liberate  H^S,  producing 
the  "curative  muds"  of  the  Black  Sea  and  other  localities.  Such 
muds  are  saturated  with  hydrogen  bisulphid  gas.  This  reduction 
of  sulphates  is,  in  a  way,  comparable  to  denitrification  since  it  is 
the  result  of  deoxidation  and  since  it  also  destroys  substances  that  are 
already  plant  foods.  Some  species  of  bacteria  appear  able  to  attack 
pure  sulphur,  causing  it  to  combine  with  hydrogen  as  H^S.     From 


ii6 


BACTERIA   AND    SOIL    MINERALS. 


these  several  sources  much  sulphur  is  liberated  into  the  air  in  the 
form  of  H2S,  the  liberation  in  all  cases  being  due  to  the  action  of 
bacteria,  difiFerent  classes  acting  on  different  compounds  containing 
sulphur. 

Recombination  of  Sulphur  Compounds. — The  recombination 
of  the  H2S  to  form  sulphuric  acid  and  then  sulphates  is  brought  about 
in  some  cases  apparently  by  purely  chemical  forces,  since  the  gas 
will  easily  combine  with  oxygen  if  the  conditions  aie  right.  But 
a  large  part  of  it  enters  new  combinations  through  the  agency 
of  microorganisms.  There  is  a  group  of  bac- 
teria that  consume  H^S,  oxidising  the  gas  within 
their  bodies  and  utilizing  the  energy  thus  liber- 
ated for  their  own  life  energy.  They  are  as 
dependent  upon  the  presence  of  H2S  as  ordinary 
plants  are  dependent  upon  COj.  In  the  pres- 
ence of  this  gas  they  flourish,  and  as  they  oxidize 
the  gas  the  sulphur  is  set  free  from  its  combina- 
tion with  hydrogen  and  separated  as  pure  sulphur. 
The  sulphur  appears  then  within  the  bodies  of 
the  bacteria  as  minute  reddish  dots  (Fig.  28). 
The  bacteria  that  can  perform  this  function  seem 
to  be  of  two  types,  one  type  belonging  to  the  higher  fungi  (see  page 
12)  and  the  other  being  true  bacteria.  The  latter  are  sometimes 
called  the  "red  bacteria"  because  of  the  color  produced  by  the  sul- 
phur grains  within  them.  These  bacteria  may  continue  thus  to  liber- 
ate the  sulphur  and  in  waters  where  H2S  is  abundant  large  quantities 
of  pure  sulphur  may  be  deposited.  These  are  the  so-called  sulphur 
springs  around  which  deposits  of  sulphur  may  be  found.  As  long  as 
the  gas  is  abundant  the  bacteria  flourish;  but  if  the  gas  disappears 
they  appear  to  use  up  the  sulphur  in  their  own  bodies,  after  which 
they  die.  In  some  way  the  sulphur  in  their  bodies  is  in  the  end 
converted  into  sulphuric  acid,  which  then  combines  with  any  lime 
that  may  be  present  to  form  sulphate  of  lime.  Very  little  is  known 
as  to  when  or  how  the  sulphur  in  the  cell  walls  of  these  sulphur 
bacteria  gets  converted  into  sulphuric  acid,  or  whether  it  is  a  purely 
chemical  or  a  biological  phenomenon.     But  although  the  whole 


Fig.  28.— Sulphur 
bacteria.  A,  Beg- 
giatoa;  B,  Ophido- 
monas.  Both  show 
sulphur  masses  in  the 
rods  {Winogradsky). 


IRON.  117 

process  is  not  fully  understood,  it  is  evident  that  microorganisms 
have  a  close  relationship  to  the  transformations  of  sulphur  in  the 
waters  and  soils.  They  liberate  it  from  its  combination  in  proteid, 
they  oxidize  the  liberated  gas  into  sulphur  and  finally  into  the 
form  of  sulphuric  acid  which  soon  forms  a  sulphate.  It  is  also 
claimed  that  some  kinds  of  bacteria  can  oxidize  sulphites  into 
sulphates.  Microorganisms  are  thus  responsible  for  the  constant 
metamorphosis  of  sulphur  compounds  that  keeps  the  soil  properly 
supplied  with  this  element.  Of  their  activities  in  ordinary  cultivated 
soil  we  know  little  or  nothing. 

IRON. 

A  small  quantity  of  iron  is  needed  by  plants,  and  a  group  of 
bacteria,  called  iron  bacteria,  has  been  supposed  to  have  some  rela- 
tion to  a  circulation  of  iron  nature,  somewhat  similar  to  that  of 
sulphur.  These  bacteria  are  commonly  seen  covered  with  a  deposit 
of  hydroxid  of  iron,  giving  them  a  reddish-brown  color.  It  has 
been  thought  that  they  used  iron  in  their  activities  much  as  sulphur 
uses  iron.  But  the  most  recent  work  indicates  that  this  is  probably 
an  error,  and  makes  the  agency  of  bacteria  doubtful.  At  all 
events,  nothing  reliable  is  known  to-day  upon  the  subject,  although 
it  is  not  impossible  that  here,  too,  the  bacteria  of  the  soil,  and 
especially  of  waters,  may  be  of  some  significance. 


CHAPTER  IX. 

SOME  PRACTICAL  LESSONS  FROM  SOIL  BACTERIOLOGY. 

The  close  dependence  of  soil  fertility  upon  the  action  of  micro- 
organisms is  manifest,  and  it  is  evident  that  farm  processes  should  be 
such  as  to  stimulate  desired  bacterial  action  and  check  these  activi- 
ties that  are  detrimental.  Practical  methods  of  doing  this  have  been 
only  partly  devised  and  there  are  still  many  problems  for  the  future 
concerning  the  method  of  treating  soil.  Nevertheless,  the  knowl- 
edge of  bacterial  action  has  already  taught  some  definite  and  useful 
lessons.  The  uncertainty  still  attached  to  certain  phases  of  the 
subject  may  be  illustrated  by  a  recently  discovered  fact  that  the 
sterilizing  of  certain  unfertile  soils  will  decidedly  increase  their 
fertility.  This  has  been  proved  definitely,  but  the  meaning  of  the 
fact  is  still  obscure.  Bacterial  action  is  positively  needed  in  the  soil, 
and  it  is  rather  surprising  that  sterilizing  soil  will  increase  its  fertility. 
It  has  been  suggested  that  the  treatment  kills  injurious  bacteria, 
giving  the  beneficial  species  that  subsequently  get  in  a  better  op- 
portunity for  growth.  It  has  been  suggested  likewise  that  the 
sterilization  kills  all  animals  and  plants  that  m.ay  be  in  the  soil,  thus 
giving  the  bacteria  that  subsequently  get  into  the  soil,  or  that  may 
have  resisted  the  sterilization,  an  extra  amount  of  organic  matter  to 
decompose  and  to  reconvert,  by  nitrification,  to  nitrate.  The  fact  of 
this  beneficial  influence  of  sterilization  is  undoubted,  although  its 
explanation  is  uncertain;  and  the  phenomenon  is  here  mentioned  as 
an  illustration  of  the  gaps  still  existing  in  our  knowledge  of  soil 
bacteriology.  But  in  spite  of  it  all,  some  definite  conclusion  as  well 
as  practical  lessons  can  already  be  drawn.* 

*  Reference  should  be  made  here  to  the  conception  concerning  soil  fer- 
tility held  by  some,  notably  those  connected  with  the  Bureau  of  Soils  of 
the  Department  of  Agriculture,  that  the  primary  trouble  in  "worn-out 
soils"  is  not  lack  of  sufficient  plant  food,  but  the  presence  of  poisonous 
excretions  that  prevent  the  growth  of  plants.     It  is  claimed  that  each  crop 

ii8 


SOIL   INOCULATIONS.  II9 

SOlL  INOCULATIONS. 

The  need  of  bacterial  action  in  the  soil  naturally  suggests  the 
question  whether  the  necessary  activities  may  not  be  brought  about 
or  increased  by  inoculating  the  soil  with  the  desired  bacteria,  just  as 
a  brewer  inoculates  his  malt  with  yeasts.  This  plan  has  been  tried 
extensively  by  at  least  two  different  methods. 

Nitrogen  Fixers,  Alinit,  Nitrifying  Bacteria. — Alinit  is  a 
material  placed  on  the  market  and  widely  used  for  a  time.  It  was 
said  to  be  made  from  a  pure  culture  of  the  bacteria  that  can  fix  free 
nitrogen  in  the  soil.  Careful  testing,  however,  failed  to  show  any 
favorable  results  from  the  use  of  alinit,  and  so  it  has  been  abandoned. 
Other  nitrogen  fixers  have  also  been  tested  as  pure  cultures  inocu- 
lated into  soil,  with  like  failure.  The  attempt  to  simulate  nitrifica- 
tion by  the  use  of  cultures  of  nitrifiers  has  likewise  failed. 

Tubercle  Bacteria  of  Legumes. — Nitragin. — As  already 
noticed,  this  commercial  product,  supposed  to  contain  the  bacteria- 
producing  root  tubercles,  was  found  to  be  a  failure,  although  claims 
are  still  made  of  the  success  attending  the  use  of  the  New  Nitragin 
and  of  some  of  the  other  cultures  of  similar  organisms.  While 
these  may  be  found  practical  and  while  it  would  seem  as  if  this 
method  would  be  likely,  in  the  future,  to  become  successful,  at  the 
present  time  the  results  are  so  uncertain  that  a  decision  as  to  the 
value  of  such  inoculation  for  the  developing  of  legume  tubercles  must 
be  held  in  abeyance.  Inoculations  with  legume  earth,  however,  have 
proved  to  be  of  great  value,  and  whenever  it  is  desired  to  cultivate 
a  legume  in  a  soil  where  it  does  not  readily  grow,  this  inoculative 

excretes  into  the  soil  certain  substances  that  serve  as  poisons  to  another 
similar  crop  on  the  same  soil.  It  is  said  that  practically  all  soils  at  all  times 
contain  a  sufficiency  of  food,  but  that  the  accumulation  of  these  excre- 
tions after  a  time  renders  the  soil  incapable  of  supporting  a  satisfactory 
crop.  The  value  of  fertilizers  is  not  to  give  food,  but  to  neutralize  these 
excretions,  and  that  a  proper  rotation  of  crops  will  serve  just  as  well,  since 
the  excretions  from  one  kind  of  plant,  while  injurious  to  the  same  plant, 
will  not  injure  a  different  kind  of  plant.  Such  a  conception  would  largely 
revolutionize  the  methods  of  treating  the  soil,  since,  if  accepted,  it  would 
lead  to  the  abandonment  of  any  attempt  to  feed  the  crops  and  would  re- 
place such  methods  by  those  designed  simply  to  remove  the  poisonous 
excretions.  This  theory  as  to  soil  fertility  would  bring  into  greater 
prominence  the  agencies  of  soil  bacteria,  but  it  is  very  vigorously  disputed 
and  certainly  has  not  reached  a  position  where  it  warrants  an  acceptance. 


I20       SOME    PRACTICAL    LESSONS    FROM    SOIL    BACTERIOLOGY. 

method  should  always  be  tried.  Soil  from  some  locality  where  the 
legume  grows  luxuriantly  should  be  imported  and  mixed  with  the 
field  which  is  to  be  planted  with  the  legume.  But  one  should  be 
careful  that  the  inoculating  soil  does  not  bring  troublesome  weeds  or 
plant  diseases.  In  using  either  the  earth  inoculations  or  pure 
cultures,  it  must  be  borne  in  mind  that  if  a  soil  is  already  well  stocked 
with  the  tubercle  bacteria,  inoculations  are  not  likely  to  do  any  good; 
but  in  soils  where  a  new  legume  is  to  be  grown  or  where  the  legume 
to  be  planted  does  not  flourish,  soil  inoculation  may  be  of  decided 
advantage. 

Soil  Inoculation  with  Manure. — Manure  is  added  to  the  soil 
primarily  as  a  means  of  furnishing  plant  food;  but  it  has  become 
evident  that  the  inoculation  of  the  soil  with  the  immense  amount  of 
bacteria  in  the  manure  is  in  itself  of  extreme  value,  sometimes,  as  in 
pasture  soil,  of  more  value  than  the  actual  food  substances  in  the 
manure.  The  use  of  manure  upon  the  soil  must,  therefore,  be 
looked  upon  as  one  of  the  very  useful  methods  of  inoculating  the 
soil  with  the  bacteria  needed  to  carry  out  the  soil  transformations. 

CONTROL  AND  STIMULATION  OF  SOIL  BACTERIA. 

It  has  become  more  and  more  evident,  as  information  has  ac- 
cumulated, that  a  vigorous  activity  of  the  bacteria  found  in  the  soil 
is  needed  to  carry  out  the  various  transformations  of  plant  foods. 
These  bacteria  are  commonly  abundant  enough;  and  sometimes 
they  find  the  conditions  so  favorable  that  they  grow  rapidly,  pro- 
ducing vigorous  actions;  but  at  other  times  the  conditions  in  the 
soil  are  unfavorable,  and  they  are  held  in  check.  What  is  chiefly 
needed,  then,  in  the  treatment  of  soil  is,  not  the  inoculation  of  more 
bacteria,  but  a  modification  of  the  soil  conditions  so  as  to  favor  the 
growth  of  those  already  there.  While  this  is  a  complicated  sub- 
ject and  one  that  will  require  different  treatment  in  different  cases, 
a  few  general  principles  may  be  formulated. 

Acidity. — Most  bacteria,  and  practically  all  the  useful  bacteria, 
are  very  sensitive  to  the  presence  of  acid,  failing  to  grow  at  all  in 
an  acid  medium.     If  the  soil  is  but  slightly  acid,  bacterial  agencies 


CONTROL  AND    STIMULATION    OF    SOIL   BACTERLA.  121 

are  checked,  while  the  activities  of  molds  and  larger  fungi  are 
increased. 

In  the  soils  of  forests,  for  example,  the  fungi  and  molds  grow 
luxuriantly,  but  bacterial  action  is  comparatively  slight.  While  the 
higher  fungi  are  valuable  agents  in  bringing  about  the  decomposi- 
tion of  certain  organic  bodies,  and  are  therefore  useful,  they  cannot 
perform  the  final  transformations  by  which  the  soil  ingredients  become 
available  as  plant  foods.  These  transformations,  especially  nitrifica- 
tion, require  bacterial  growth.  Hence,  it  follows  that  one  of  the  first 
necessities  of  proper  bacterial  activity  is  an  alkaline  reaction  in  the 
soil.  In  some  localities  this  matter  cares  for  itself.  If  the  soil 
contains  lime  in  any  form,  the  solution  of  lime  by  the  carbonated 
water,  resulting  from  the  carbonic  dioxid  of  decomposition,  will 
keep  the  soil  properly  alkaline.  Decomposition  in  itself  will  also 
produce  an  alkaline  condition,  since  the  ammonia  resulting  from 
ammoniacal  fermentation  will  neutralize  the  acids.  If,  therefore, 
a  vigoious  decomposition  of  organic  matter  is  going  on,  little 
attention  need  be  given  to  the  matter  of  acidity.  But  some  soils 
are  acid  from  one  cause  or  another,  and  proper  bacterial  activities 
cannot  be  expected  here  without  the  correction  of  this  acidity. 
This  is  most  easily  done  by  the  addition  of  lime,  either  in  the  form 
of  limestone,  plaster,  ground  shells,  or  some  other  common  substance. 
The  restoration  of  alkaline  reaction  will  be  followed  by  a  stimulation 
of  bacterial  activities  and  an  increased  fertility. 

Aeration. — The  soil  bacteria  are  aerobic  and  anaerobic,  and  both 
types  are  sometimes  useful  and  sometimes  detrimental.  Speaking 
broadly,  however,  the  aerobic  processes  in  the  soil  are  the  more 
desirable.  Anaerobic  decomposition  is  incomplete,  and  gives 
rise  to  many  undesirable  products,  while  aerobic  decomposition  is 
complete  and  hence  a  more  useful  process.  Nitrification,  too, 
can  go  on  only  in  the  presence  of  oxygen,  and  is  stimulated  by  a 
quantity  of  this  gas.  The  value  of  a  frequent  stirring  or  cultivating 
of  the  soil,  which  introduces  air  into  it,  is,  therefore,  evident.  The 
simple  stirring  of  the  soil,  to  bring  oxygen  into  close  contact  with  its 
bacteria,  may  be  of  as  much  value  as  an  application  of  manure.  In 
some  soils,  indeed,  it  is  more  valuable  than  manure,  since  there 


122        SOME    PRACTICAL    LESSONS    FROM    SOIL    BACTERIOLOGY. 

may  be  plenty  of  organic  products  in  the  soil  which  only  need  trans- 
forming in  order  to  be  available  for  plants.  All  desirable  processes 
which  are  likely  to  occur  in  the  soil  are  benefited  by  aeration.  In 
especially  rich  collections  of  organic  refuse,  however,  like  the  manure 
heap,  aeration  will  cause  large  losses  through  denitrification,  and 
hence  the  manure  should  be  closely  packed  to  exclude  air.  This 
may  be  true  also  in  some  instances  of  intensive  gardening,  where 
large  amounts  of  manure  are  applied  to  the  soil  as  top  dressing. 
But  in  ordinary  soil,  aeration  from  frequent  stirring  stimulates 
desirable  bacterial  activities. 

Manures  Better  than  Commercial  Fertilizers. — This  perfectly 
evident  conclusion  is  of  so  much  importance  as  to  deserve  special 
emphasis.  The  reasons  for  the  conclusion  are  several.  Manure 
adds  bacteria  as  well  as  chemical  food.  It  helps  to  keep  a  proper 
alkalinity  in  the  soil.  It  adds  various  ingredients  that  help  to  form 
humus  in  the  soil,  resulting  in  a  better  texture  and  more  lasting 
good.  Manure  adds  to  the  soil  in  small  amounts  various  useful 
materials,  which  are  not  present  in  commercial  fertilizers.  For 
these  cogent  reasons  manure  fertilizers  are  to  be  preferred,  as  a 
rule,  to  chemical  fertilizers.  From  all  these  facts  may  be  drawn 
the  practical  lesson  that  a  properly  kept  farm  should  keep  plenty  of 
live  stock  and,  instead  of  selling  its  manure,  should  use  it  freely  on 
the  soil. 

FALLOWING. 

It  is  an  old  idea  that  the  soil,  after  yielding  several  crops,  needs 
a  rest,  an  idea  that  goes  back  as  far  as  the  Romans.  From  this  arose 
the  plan  of  occasionally  allowing  the  soil  to  remain  without  a  crop 
for  a  season  or  for  part  of  a  season.  This  plan  has  practically 
gone  out  of  use  in  all  ordinary  soils,  for  careful  study  shows  that 
a  detriment  rather  than  a  benefit  results  from  such  fallowing. 
Under  certain  conditions  fallowing  may  be  an  advantage.  There 
is  a  smaller  loss  of  water  from  fallow  land  than  from  land  with  grow- 
ing crops,  due  partly  to  the  increased  evaporation  from  the  stirred 
soil  and  partly  to  the  fact  that  crops  draw  quantities  of  water  from 
the  soil,  to  be  evaporated  from  the  growing  leaves.     In  climates 


GREEN    MANURING. 


123 


where  water  is  scarce  and  must  be  conserved,  fallowing  may  result 
in  advantage.  It  is  also  claimed  that  fallowing  may  enable  the 
soil  to  dispose  of  the  poisonous  secretion  from  plants  that  would 
injure  a  second  crop  growing  on  the  same  soil.  But,  apart  from 
these  facts,  fallowing  results  in  a  loss  to  the  soil.  In  the  first  place, 
fallowing  adds  nothing  to  the  soil,  while  a  crop,  especially  a  legume, 
may  do  so.  Moreover,  during  the  fallow  season  the  bacterial  activi- 
ties in  the  soil  continue,  converting  the  material  in  the  humus 
into  nitrates  and  other  soluble  substances,  which  are  then  available 
plant  foods.  If  a  crop  is  growing  in  the  soil  these  will  be  absorbed 
by  the  crop  and  utilized.  If,  however,  the  land  is  fallow,  there  is 
nothing  to  utilize  these  products  as  they  are  formed,  and  they  will 
be,  in  a  measure,  lost;  for  they  will  be  dissolved  in  the  soil  waters 
and  drained  away  from  the  soil  into  the  general  system  of  brooks  and 
streams.  If,  in  the  meantime,  nothing  of  any  value  is  added  to 
the  soil,  at  the  end  of  the  fallowing  it  will  actually  be  poorer  than  at 
the  beginning,  except  in  the  matter  of  water.  Its  store  of  humus 
will  be  partly  converted  into  available  plant  food  and  lost,  while 
nothing  takes  its  place.  For  these  reasons  the  practice  of  fallowing 
has  been  almost  wholly  given  up,  except  for  special  soils.  Indeed, 
it  is  a  growing  custom  not  to  allow  the  soil  to  remain  fallow  at  all, 
not  even  during  the  season  of  the  year  when  the  main  crop  is  not 
growing;  but  to  sow  it  with  a  cover  crop,  which  will  catch  and  hold 
the  plant  foods  that  are  constantly  being  made  available.  Nitrifica- 
tion, as  we  have  seen,  goes  on  at  all  seasons  when  the  soil  is  not 
actually  frozen,  and  considerable  losses  of  nitrogen  will  result  from 
the  leaching  of  these  nitrates  away  from  the  soil  at  the  seasons 
when  it  is  not  covered  with  a  crop.  The  loss  may  be  largely  retained 
by  a  quickly  growing  cover  crop.  Such  cover  crops  plowed  into 
the  soil  will  benefit  it,  and  the  next  main  crop  will  be  improved; 
but  a  fallow  season  leads  to  direct  loss. 

GREEN  MANURING. 

The  use  of  cover  crops,  just  mentioned,  is  closely  related  to  the 
practice  of  green  manuring,  but  the  latter  has  an  additional  purpose 


124       SOME    PRACTICAL    LESSONS    FROM    SOIL    BACTERIOLOGY. 

besides  all  the  advantages  of  a  cover  crop.  It  not  only  prevents 
the  loss  of  available  plant  foods  by  drainage,  but  it  also  adds  a 
considerable  quantity  of  food  to  the  soil.  It  not  only  serves  as  a 
catch  crop  to  hold  the  nitrates  that  may  form  during  the  season 
when  the  main  crop  is  not  growing,  but  it  may  add  to  the  total  nitro- 
gen of  the  soil.  The  general  plan  of  green  manuring  is  to  grow  upon 
the  soil  some  leguminous  crop  that  increases  the  nitrogen  content 
of  the  soil,  and,  after  proper  growth,  to  plow  the  whole  crop  into  the 
soil.  The  addition  of  this  large  amount  of  organic  matter  to  the  soil 
will  stimulate  the  bacterial  activities  of  decomposition.  The  roots, 
stems,  leaves,  and  fruits  of  the  crop  undergo  a  decomposition  and 
subsequent  nitrification,  resulting  finally  in  the  formation  of  nitrates 
which  can  be  utilized  by  the  next  crop  grown  on  the  same  soil. 

When  adopting  the  plan  of  green  manuring  the  first  thing  is  to 
select  the  crop  that  is  to  be  so  utilized.  Manifestly,  from  what  has 
been  learned,  this  should  be  some  one  of  the  legumes,  since  this  family 
of  plants  alone  assimilates  nitrogen  from  the  air,  at  least  in  any  con- 
siderable quantity.  Other  plants  have  been  used  for  the  purpose, 
but  while  they  are  valuable  in  supplying  some  organic  material 
which  helps  maintain  the  store  of  humus,  they  are  far  inferior  to 
legumes  which,  in  addition  to  all  the  other  advantages,  add  usable 
nitrogen  in  quantity.  Attention  should  be  given  to  the  nature  of 
the  soil,  and  to  the  kind  of  legume  that  will  best  flourish  in  the  soil ; 
the  legume  must  produce  plenty  of  root  tubercles,  otherwise  the 
chief  value  of  the  green  manuring  is  lost.  Green  manuring  is  of 
particular  value  in  sandy,  loose  soils,  where  the  humus  is  scanty, 
and  where  the  texture  of  the  soil  facilitates  losses  by  draining.  In 
such  soils  so  rapid  is  the  draining  that  it  is  sometimes  difficult  to  get 
fertilizers  to  remain  in  the  soil  long  enough  for  their  proper  assimila- 
tion by  the  plant.  The  use  of  legumes,  plowed  under  to  furnish  a 
mass  of  decaying  vegetation,  greatly  improves  the  texture  of  the  soil 
and  will,  in  time,  give  them  a  fair  humus  content.  By  this  means 
very  unpromising  sandy  soils  can  be  reclaimed  to  a  fair  condition  of 
fertility.  The  legumes  found  to  be  best  adapted  to  such  sandy  soils 
are  the  cow  pea,  the  soy  bean,  the  velvet  bean  and  the  crimson  clover. 

With  clay  soils,  on  the  other  hand,  green  manuring  must  be 


GREEN   MANURING.  1 25 

handled  somewhat  differently.  The  density  of  the  soil  reduces  the 
ordinal y  losses  by  drainage,  so  that  there  is  less  need  of  the  green 
manuring.  The  legumes  found  most  useful  on  such  lands  are 
lupins,  seradellay  yellow,  red  and  crimson  clover,  field  peas,  horse 
beans  and  vetches.  Because  of  the  density  of  the  soil,  decomposition 
does  not  progress  so  freely;  hence  care  must  be  taken  not  to  overdo 
the  treatment  by  plowing  in  too  much  of  the  green  plant. 

The  extent  of  the  utilization  of  the  legume  after  growing,  depends 
upon  the  completeness  of  the  decomposition  and  the  eventual  nitrifi- 
cation of  the  material  that  is  plowed  under.  It  is  possible  to  plow 
under  such  a  large  quantity  of  vegetable  matter  that  it  will  not 
properly  decay,  either  because  the  density  of  the  soil  prevents  suflBi- 
cient  aeration,  or  because  too  much  acid  forms,  checking  bacterial 
activities.  No  value  accrues  from  green  manuring  unless  thorough 
decomposition  occurs.  For  this  reason  it  is  generally  best  to  plow 
in  the  leguminous  crop  before  it  has  fully  matured,  for  then  it  has 
assimilated  most  of  its  nitrogen  but  has  not  become  too  bulky  for 
proper  decay  in  the  soil.  Frequently  it  is  best  to  reap  the  crop  and 
feed  it  to  cattle,  plowing  in  only  the  roots  and  stubble,  this  giving  all 
the  organic  matter  that  can  be  readily  decomposed  in  the  soil.  If, 
subsequently,  the  manure  from  the  cattle  that  eat  the  crop  is  added 
as  a  dressing,  the  greatest  possible  use  will  have  been  made  of  the 
leguminous  crop. 

The  actual  value  of  such  green  manuring  has  been  demonstrated 
many  times.  Sandy  soils  have  been  brought  under  fair  cultivation, 
and  depleted  farms  have  been  reclaimed  to  cultivation  by  the  skill- 
ful use  of  this  nitrogen-fixing  power  of  legumes,  aided  by  the  tubercle 
bacteria.  A  constant  increase  in  nitrogen  can  be  brought  about 
thus  till  the  quantity  is  sufficient  for  large  crops.  In  one  extended 
series  of  experiments  and  observations  it  was  found  possible  to  in- 
crease the  amount  of  nitrogen  in  a  soil  from  .02  per  cent,  at  the  start, 
to  .17  per  cent,  at  the  end  of  about  twenty-five  years,  equivalent  to 
5,000  pounds  of  nitrogen  per  acre.  In  another  test  the  plowing  in  of 
a  crop  of  the  velvet  beans  stubble  upon  a  soil  subsequently  planted 
with  oats,  increased  the  yield  of  oats  from  seven  to  thirty-eight 
bushels  per  acre;  and  in  this  case  the  velvet  bean  crop  was  reaped 


126        SOME    PRACTICAL    LESSONS    FROM    SOIL    BACTERIOLOGY. 

and  utilized,  only  the  roots  and  stubble  being  necessary  for  the  in- 
creased yield. 

The  great  lesson  to  be  drawn  from  this  subject  is  that,  by  means 
of  the  nitrogen-fixing  power  of  the  legumes,  aided  by  bacteria  in 
their  roots,  the  farmer  has  a  practical  means  of  maintaining  the 
nitrogen  content  of  his  soil  at  a  proper  degree  for  high  fertility. 
There  is  no  need  of  purchasing  nitrate  fertilizers.  The  money  may  be 
better  spent  on  phosphorus  and  potash.  The  cultivation  of  legumes 
seems  to  be  the  secret  of  the  continuance  of  agriculture,  and  if  the 
farmer  will  only  learn  the  principles  and  acquire  the  habit  of  alter- 
nating legumes  with  his  other  crops,  he  may  maintain  indefinitely 
a  high  fertility  in  his  soil,  in  spite  of  long-continued  cultivation. 
When  in  addition  to  this  we  remember  the  fact  that  the  soil  minerals 
are  being  constantly  dissolved  in  the  soil  waters,  it  becomes  evident 
that  the  farmer  is  by  no  means  as  dependent  upon  artificial  fertilizers 
as  has  been  supposed. 


CHAPTER  X. 

BACTERIA  IN  WATER. 

This  subject  is  of  great  importance  in  the  relation  it  bears  to  the 
water  supplies  of  cities,  and  most  of  its  important  phases  concern 
only  the  city  water-supply.  So  far  as  relates  to  farm  life  the  sub- 
ject has  interest  in  two  directions:  i.  The  purity  of  the  drinking- 
water.     2.   The  pollution  of  streams. 

ABUNDANCE  OF  BACTERIA  IN  WATER. 

All  surface  waters  contain  bacteria.  We  shall  find  them  when- 
ever we  examine  the  water  of  the  ocean,  the  brook,  the  pool  or  the 
reservoir.  Even  rain  water  contains  them,  doubtless  washed  from 
the  air,  and  the  same  is  true  of  snow  and  hail. 

The  number  of  bacteria  in  water  is  not  exactly  what  would  be 
expected  in  accordance  with  our  ideas  of  pure  water.  The  water  in 
the  running  brook  is  commonly  thought  of  as  purer  than  that  of  the 
stagnant  pond.  But  this  is  certainly  not  true;  the  brook  contains 
more  bacteria  than  the  pond,  and  the  supply  streams  always  contain 
more  bacteria  than  the  water  of  the  lake  or  reservoir.  The  reason 
for  this  is  evident.  The.  brooks  form  the  drainage  system  of  the 
country.  The  rains  wash  the  whole  surface  of  the  land,  and  all  the 
dirt  and  dust  is  carried  into  the  brooks.  In  this  dust  will  always  be 
hosts  of  bacteria  which  are  thus  carried  by  the  streams  into  the  lake 
in  great  numbers.  In  the  lake  many  of  them  soon  die;  others 
settle  to  the  bottom;  the  water  in  the  reservoir  rapidly  becomes 
purified,  and  it  always  contains  fewer  bacteria  than  the  water 
brought  into  it  by  its  supply  streams. 

The  number  of  bacteria  in  a  body  of  water  will  depend  upon  the 
extent  of  the  contamination  which  it  receives  from  sources  of  active 
bacterial  growth.     The  actual  number  is  quite  variable,  ranging 

127 


128  BACTERIA   IN    WATER. 

from  a  score  or  more  per  c.c.  in  very  pure  waters  to  a  few  hundreds 
in  a  moderately  pure  reservoir;  and  from  this  number  to  many 
thousands  in  streams  which  are  badly  contaminated  with  sewage. 

I.  THE  PURITY  OF  DRINKING-WATERS. 

In  determining  the  purity  of  drinking-water  we  are  not  so  much 
concerned  with  the  number  of  bacteria  it  contains  as  with  the  kinds. 
Very  large  numbers  may  be  present  and  yet  the  water  may  be 
perfectly  wholesome,  while,  on  the  other  hand,  with  only  a  small 
number  present  the  water  may  be  deadly.  As  we  shall  see  in  a 
later  chapter,  the  bacteria  in  milk  may  be  reckoned  by  the  millions 
per  c.c,  and  yet  the  milk  may  be  perfectly  healthful;  and  at  the  same 
time  bacteriologists  regard  with  suspicion  water  that  contains  them 
only  in  thousands.  The  reason  for  this  difference  is  simple.  When 
milk  contains  these  large  numbers  they  are  almost  sure  to  be  harmless 
types;  but  if  water  contains  even  a  few  thousands,  the  typhoid 
bacillus  is  likely  to  be  among  them.  It  is  impossible  to  condemn 
any  sample  of  water  simply  from  the  number  of  bacteria  which 
it  contains;  still  the  number  serves  as  a  useful  measure  of  purity 
for  the  following  reason:  Water  that  is  fairly  pure  and  contains 
only  the  bacteria  liable  to  come  from  ordinary  sources  seldom  contains 
more  than  a  few  hundreds  of  bacteria  per  c.c;  it  is  only  water 
that  is  receiving  contamination  from  sewage  or  some  other  source 
of  decaying  filth  that  contains  large  numbers  of  bacteria.  Hence, 
the  finding  of  large  numbers  of  bacteria  in  a  water-supply  suggests 
sewage  contamination  and  the  water  at  once  becomes  suspicious. 

SEWAGE  CONTAMINATION. 

Water  may  receive  hosts  of  bacteria  from  various  sources,  but 
the  one  great  and  almost  only  source  of  real  danger  is  from  sewage 
contamination.  Most  of  the  types  of  bacteria  found  in  nature 
and  in  natural  water  are  perfectly  harmless,  so  that  it  makes  little 
difference  whether  they  are  abundant  or  few  in  the  water  we  drink. 
But  this  is  not  true  of  the  types  likely  to  be  found  in  sewage.     Sew- 


THE    PURITY    OF    DRINKING-WATERS.  I29 

age  contains  every  form  of  human  excretions;  and  since  it  is  by 
means  of  the  excretions  that  the  pathogenic  bacteria  find  exit 
from  the  diseased  patient,  it  will  thus  be  easily  understood  that 
sewage-contaminated  water  is  likely  to  contain  bacteria  which  are 
pathogenic  for  man.  Such  water  is  therefore  always  dangerous, 
a  fact  abundantly  proved  by  the  great  prevalence  of  water-borne 
diseases  in  cities  whose  water-supply  is  contaminated  with  sewage. 
When  the  bacteria  in  water  are  in  the  thousands  per  c.c,  they  render 
the  water  unsafe,  not  because  this  number  of  bacteria  is  injurious, 
but  because  such  water  is  commonly  sewage-contaminated. 

When  we  recognize  the  great  chance  which  sewage-contaminated 
water  has  of  becoming  impregnated  with  the  germs  of  human 
diseases,  it  is  a  little  surprising  to  learn  that  the  number  of  kinds  of 
disease  actually  distributed  by  water  is  very  small.  Only  one  of 
our  common  diseases  is  known  to  be  frequently  distributed  by 
water.  This  is  typhoid  fever,  in  regard  to  which  the  evidence  is 
abundant  and  conclusive.  This  evidence  need  not  be  given  here, 
but  it  is  sufficient  to  demonstrate  that  typhoid  fever  is  very  commonly 
acquired  from  drinking-water,  that  the  danger  comes  wholly  from 
water  which  has  in  some  way  become  contaminated  with  human 
excrement,  usually  through  sewage,  and  that  the  drinking  of  sewage- 
contaminated  water  is  probably  the  most  prolific  source  of  this 
dreaded  and  serious  disease. 

Other  water-borne  diseases  are  of  less  importance.  Asiatic 
cholera  is  distributed  by  water,  but  this  is,  at  least  in  this  country, 
of  no  significance.  Certain  forms  of  dysentery  are  probably  distrib- 
uted by  water,  but  little  is  known  of  this  matter  as  yet.  No'other 
diseases  are  known  to  be  thus  distributed. 

Detection  of  Sewage  Contamination. — Sewage  contamination 
is  a  rapidly  growing  danger.  As  population  increases,  the  amount 
of  sewage  also  increases,  and  it  becomes  more  and  more  difficult 
to  dispose  of  it  so  as  to  prevent  its  contaminating  the  sources  of 
drinking-water.  Many  a  stream  formerly  used  for  drinking  purposes 
has  had  to  be  abandoned  because  it  has  become  so  polluted  with 
sewage  as  to  be  no  longer  safe.  It  is  thus  a  matter  of  prime  necessity 
to  find  some  delicate  means  of  determining  whether  water  is  sewage 


130  BACTERIA    IN    WATER. 

contaminated;  for  while  sometimes  the  contamination  is  so  great  as  to 
be  evident,  in  most  cases,  especially  in  wells,  it  cannot  be  detected 
by  ordinary  examinations.  The  chemical  analysis  of  water  gives  no 
sure  indication,  and  the  determination  of  the  number  of  bacteria 
alone  is  only  suggestive.  It  chances,  however,  that  there  is  a 
species  of  bacterium  called  B.  coli,  that  is  a  common  inhabitant  of 
the  human  intestine,  but  is  rarely  found  free  in  nature  or  inhabiting 
pure  waters.  This  B.  coli  is  so  abundant  in  feces  that  it  is  practically 
sure  to  be  found  in  all  sewage-contaminated  waters,  while  it 
is  not  found,  at  least  to  any  great  extent,  in  water  free  from  sewage 
contamination.  Since  this  bacillus  is  fairly  easy  to  recognize  by 
bacteriological  methods,  it  is  not  difficult  to  determine  whether 
or  not  it  is  present  in  a  sample  of  water.  Hence  this  bacterium 
becomes  a  test  for  sewage  pollution.  A  sample  of  water  showing 
the  presence  of  B.  coli  is  almost  surely  contaminated  by  sewage, 
while  water  free  from  it  is  not  thus  polluted.  The  report  from  a 
bacteriologist  that  B.  coli  is  found  means,  then,  that  the  water  is 
unsafe,  since  sewage  contamination  may  at  any  time  infest  it  with 
typhoid  germs.  While  B.  coli  itself  is  harmless,  its  presence  indi- 
cates the  certainty  of  danger. 

PURITY  OF  DRINKING-WATER  FROM  DIFFERENT  SOURCES. 

Recognizing  sewage  contamination  as  the  great  source  of  danger 
in  drinking-water,  we  may  classify  waters  as  pure  or  safe  in  propor- 
tion to  their  freedom  from  such  contamination. 

Water  from  Streams. — Ordinary  streams  are  the  most  likely 
to  be  sewage  contaminated.  They  constitute  the  drainage  system 
of  the  land,  receiving  sewage  from  towns,  villages,  and  cities.  The 
amount  of  sewage,  and  hence  the  extent  of  the  danger,  depends  upon 
the  number  of  people  contributing  to  produce  it  and  upon  the  size 
of  the  stream.  The  only  safe  position  to  hold,  however,  is  that 
all  streams  upon  whose  banks  are  human  habitations  are  polluted 
and  unsafe  for  drinking.  The  question  of  the  purification  of  such 
water  will  be  noticed  later. 

Wells. — Next  to  running  streams,  wells  are  the  most  dangerous 
source  of  drinking-water.     The  extent  of  the  danger  depends  upon 


THE    PURITY    OF    DRINKING-WATERS.  I3I 

the  location  of  a  well  and  its  depth.  In  very  deep  wells  bacteria 
have  a  chance  to  be  filtered  out  of  the  water  as  it  passes  through  the 
soil  before  it  reaches  the  well,  so  that,  if  care  be  taken  to  prevent 
contamination  at  the  surface,  the  water  is  safe.  This  is  always  true 
of  artesian  wells.  But  in  the  shallow  well  the  chance  of  dangerous 
contamination  is  great.  The  most  common,  as  well  as  the  most 
dangerous  contamination  of  well-water,  comes  from  the  privy  vault. 
Both  vault  and  well  are,  for  convenience,  placed  near  the  house  and 
frequently  near  each  other.  The  well  is  sunken  several  feet  below 
the  surface  of  the  ground,  while  the  vault  is  close  to  the  suiface. 
The  contents  of  the  vault  inevitably  soak  into  the  ground  and  will  be 
surely  distributed  in  every  direction,  taking  naturally  the  course  of 
water  currents  under  the  surface.  It  is  almost  certain  that,  if  the 
well  is  close  at  hand,  the  water  courses  will  lead  to  it  and  the  con- 
tents of  the  vault  will  thus  find  their  way  into  the  well.  It  requires 
no  argument  to  demonstrate  the  danger  from  such  conditions.  Nor 
will  anyone  familiar  with  agricultural  communities  fail  to  recognize 
that  exactly  such  conditions  frequently  exist.  Indeed,  they  are 
sometimes  even  worse  than  this,  for  one  may  find  the  vault  actually 
upon  an  elevated  mound  and  the  well  sunk  into  the  soil  at  its  foot 
not  twenty  feet  away. 

Under  such  conditions  one  need  not  be  surprised  at  the  spread  of 
typhoid.  A  single  case  of  the  disease  on  the  farm  will  contaminate 
the  vault,  and  may  soon  infect  the  well.  The  infection  may  be 
from  water  percolating  through  the  soil  or  from  surface  currents  in 
time  of  rains,  washing  the  contaminated  water  into  the  mouth  of  the 
well.  The  farmer  rinses  his  milk  pails  in  the  water  from  the  well 
and  subsequently  puts  his  warm  milk  in  the  cans.  The  typhoid 
bacilli  which  were  in  the  well  thus  get  into  the  milk,  where  they 
find  conditions  for  rapid  growth,  and  the  farmer,  wholly  unconscious 
of  having  done  anything  out  of  the  way,  distributes  the  bacilli  to  the 
neighboring  community  which  he  supplies  with  milk.  A  typhoid 
fever  epidemic  breaks  out  which  remains  a  mystery,  unless  some  one 
is  sharp  enough  to  trace  it  to  its  source  in  the  farmer's  well. 

Such  is  not  an  imaginary  instance,  but  represents  a  type  of 
typhoid  epidemic  many  times  repeated.     It  is  simply  illustrative  of 


132  BACTERIA   IN   WATER. 

one  of  the  sources  of  typhoid  epidemics  which  has  been  found  com- 
mon, and  many  instances  of  almost  exactly  these  conditions  could 
be  given.  Nearly  three  hundred  typhoid  epidemics  have  been 
traced  to  milk,  many  of  which  are  directly  attributable  to  the  well. 
The  trouble  arises  partly  from  carelessness,  but  chiefly  from  ignor- 
ance. Certainly,  for  his  own  health  and  that  of  the  community 
which  he  supplies  with  milk,  every  farmer  should  be  impressed 
with  the  fact  that  the  problem  of  his  well  is  most  critical.  It  should 
be  scrupulously  guarded,  and  should  be  located  in  such  a  place  as  to 
render  drainage  from  the  privy  vault  an  impossibility.  The  safest 
thing  would  be  to  give  up  the  well  entirely  and  depend  upon  some 
spring  or  reservoir;  but  where  this  is  impossible  the  well  should  be 
on  higher  ground  than  the  privy  vault,  or  be  removed  from  it  not 
less  than  one  hundred  feet. 

Unfortunately,  everyone  who  has  been  brought  up  on  a  farm  is 
likely  to  feel  that  this  danger  is  imaginary,  at  least  for  his  own 
particular  home.  He  has  drunken  water  from  the  well  all  his  life, 
and  so  have  his  fathers  before  him,  and  he  cannot  be  convinced  of 
any  danger  therein.  But  the  fact  remains  that  many  a  well  of 
exactly  this  sort  has  been  the  cause  of  typhoid.  Though  used  for 
years  without  suspicion  it  has,  nevertheless,  been  a  means  of  death. 
The  trouble  gives  no  warning  when  it  comes,  and  the  well  which  has 
been  pure  for  years  may  suddenly  begin  to  distribute  typhoid  fever 
bacilli  without  the  least  suspicion  on  the  part  of  those  using  it.  In 
ignorance  the  farmer  not  only  drinks  the  water  himself,  but  dis- 
tributes the  germs  to  the  city,  insisting  all  the  while  that  his  well  has 
"the  finest  water  in  the  country."  The  only  safeguard  is  either  to 
abandon  the  well  entirely,  or  to  have  such  an  absolute  isolation  be- 
tween his  vault  and  his  well  as  to  make  communication  between 
them  by  soil  drainage  an  absolute  impossibility. 

Since  the  water  in  the  well  is  likely  to  become  contaminated  with 
typhoid  bacteria,  if  excreta  are  thrown  upon  the  ground  or  are 
placed  in  a  vault  in  the  vicinity  of  the  well  which  is  used  for  drinking 
or  dairy  purposes,  especial  care  should  be  taken  that  no  surface 
rivulets  in  time  of  rain  should  run  toward  the  well.  If  they  do, 
contamination  of  the  water  by  surface  drainage  is  almost  certain. 


THE   PURITY   OF   DRINKING-WATERS.  I33 

Cisterns.— Cisterns,  to  hold  rain-water  caught  from  the  roofs  of 
houses,  have  frequently  been  used  as  a  source  of  water  and  are,  to  a 
certain  extent,  so  used  to-day,  particularly  in  localities  where  the 
natural  waters  of  the  soil  are  very  hard.  These  cisterns  are  just  as 
dangerous  as  wells;  sometimes  more  so.  They  are  generally 
placed  where  it  is  almost  sure  that  they  will  become  contaminated  in 
some  way,  and  actual  examination  of  such  cisterns  usually  shows 
the  B.  coli  present,  indicating  sewage  contamination.  Instances 
are  also  known  where  they  have  been  the  means  of  distributing 
typhoid  fever. 

Stored  Water  in  Reservoirs  or  Lakes. — These  constitute  a 
far  better  source  for  drinking-water,  and  under  ordinary  circum- 
stances are  perfectly  safe.  Even  the  water  of  a  contaminated 
stream  will  become  free  from  dangerous  disease  germs  when  it  has 
been  stored  for  a  few  weeks.  This  is  partly  because  the  bacteria 
sink  to  the  bottom,  and  are  not  likely  to  get  into  the  water  mains; 
but  it  is  chiefly  because  the  disease  germs  cannot  live  very  long  in 
water.  Typhoid  germs  cannot  live  more  than  six  weeks  (usually 
not  so  long)  in  ordinary  water,  and  if  it  be  stored  so  long  before  it  is 
used,  it  will  be  free  from  this  danger,  even  though  at  first  it  was 
sewage  contaminated.  The  stored  water  of  reservoirs  thus  con- 
stitutes the  best  large  supply  of  water.  It  may  be  something  of  a 
surprise  to  be  told  that  stored  water  is  purer  and  safer  than  running 
water,  but  study  and  experience  have  shown  this  to  be  positively  the 
case. 

Springs. — These  are  thoroughly  reliable  sources  of  drinking- 
water  if  they  are  properly  guarded.  The  water  comes  from  under- 
ground and  has  filtered  through  the  soil  for  unknown  distances. 
There  may  be  cases,  it  is  true,  where  the  filtering  is  through  only  a 
thin  layer  of  soil,  insufl&cient  to  purify.  But  if  such  cases  exist, 
they  are  very  unusual,  and  examination  shows  spring-water  to  be 
free  from  disease  germs,  unless  carelessly  contaminated  after  the 
water  leaves  the  soil.  The  spring  should  be  classed  with  the  artesian 
well  in  this  respect,  and  is  the  best  source  of  water  that  can  be 
obtained. 

Filtered   Water. — The    rapidly   extending   contamination   of 


134  BACTERIA    IN    WATER. 

waters  by  sewage  and  the  growing  demand  for  water  have  led  to 
development  of  methods  of  filtering  such  contaminated  water  in 
large  quantities.  This  is  done  by  passing  it  through  layers  of  sand 
which  are  constructed  in  such  a  way  as  to  remove  most  of  the 
bacteria.  These  filters  are  in  wide  use  to-day  by  cities  that  have 
to  depend  upon  a  contaminated  supply.  The  bacteria  are  not 
wholly  removed  from  the  water,  but  so  nearly  that  practically  all 
dangers  disappear.  Experience  has  shown  that  the  use  of  filters 
very  greatly  reduces  the  amount  of  typhoid  fever  in  cities  dependent 
upon  a  contaminated  water-supply.  It  is  also  found  that  the 
purified  water  improves  the  general  health  of  the  community,  quite 
apart  from  the  decrease  in  typhoid  fever. 

Ice. — Ice,  though  not  thought  of  as  water,  in  summer  months 
is  put  into  drinking-water  to  cool  it.  The  ice  melts  and  whatever 
bacteria  are  in  it  are  liberated  and  swallowed  with  the  water.  It 
has  been  a  belief  that  free2;ing  purifies  water,  so  that  many  have 
been  perfectly  willing  to  use  ice  from  ponds  whose  water  they  would 
not  drink.  It  is  a  very  wide  practice  to  cut  the  year's  ice  supply 
from  sewage-contaminated  streams,  and  from  places  where  no  one 
would  think  of  drinking  the  water:  e.g.,  from  the  Hudson  River, 
below  Albany.  It  has  become  a  matter  of  great  importance  to 
know  whether  freezing  does  purify  such  ice  and  render  it  safe. 
The  subject  has  been  most  carefully  investigated,  with  the  following 
conclusion.  Ice  does  in  a  measure  purify  itself  in  freezing,  but  not 
wholly.  If  typhoid  bacilli  are  in  the  water,  they  may  be  found  in 
the  loose  snow  ice  at  the  top  of  the  frozen  layer,  but  there  are  very 
few,  if  any,  in  the  clear  ice  below.  After  the  ice  has  been  stored 
for  a  while  the  typhoid  bacilli  become  less  and  less  abundant,  and 
after  a  few  weeks  they  practically  disappear.  Even  after  months 
of  freezing,  however,  a  few  may  sometimes  be  found,  so  that  no 
ice  from  contaminated  water  can  be  guaranteed  as  absolutely  free 
from  them  even  after  six  months'  storage.  But  the  number  that 
resists  this  storage  is  so  extremely  small  that  the  ice  is  as  pure  as 
filtered  water.  No  cases  of  typhoid  fever  have  been  definitely 
traced  to  such  a  source,  though  one  or  two  doubtful  cases  have 
been  so  attributed.     In  general,  then,  it  appears  that  stored  ice 


THE    CONTAMINATION   AND    PURIFICATION    OF    STREAMS.       I35 

is  safe,  provided  it  is  free  from  snow  ice  and  has  been  stored  at  least 
two  or  three  months. 


II.  THE  CONTAMINATION  AND  PURIFICATION 
OF  STREAMS. 

The  sewage  contamination  of  streams  has  been  increasing  year 
by  year,  until  many  a  stream  that  was  clear  and  limpid  thirty  years 
ago  is  now  a  vile  collection  of  filth.  Until  some  other  means  of 
disposing  of  sewage  is  generally  adopted,  this  pollution  must  continue 
to  increase.  There  has  been  a  widely  held  belief  that  running  water 
purifies  itself,  and  that  these  streams  rapidly  become  free  from 
their  pollution.  This  is  partly  correct  and  partly  erroneous.  A 
stream  does  not  purify  itself  by  running,  but  there  is  always  a 
tendency  for  water  to  become  pure,  and  in  time  sewage  contamina- 
tion quite  disappears  from  water,  whether  running  or  stagnant. 
The  best  studied  example  of  this  is  in  the  Chicago  Drainage  Canal. 
Recently  the  city  of  Chicago  converted  the  Illinois  River  into  a 
drainage  canal  for  the  great  amount  of  sewagfe  of  that  city.  This 
river  is  a  small  one  and  flows  very  slowly.  It  finally  empties  into 
the  Mississippi  River,  after  flowing  some  300  miles.  It  empties 
a  few  miles  above  the  point  where  St.  Louis  takes  its  water-supply, 
and  naturally  it  excited  considerable  alarm  in  the  latter  city.  A 
careful  examination  of  the  bacteria  in  the  river  shows  that  there  is 
a  constant  decrease  in  numbers  as  the  distance  from  Chicago  is 
increased,  and  when  it  finally  empties  into  the  Mississippi,  all  of 
the  bacterial  contamination  from  Chicago  has  disappeared.  In 
this  flow  the  river  has  purified  itself  of  sewage  bacteria.  In  other 
examples  when  the  polution  is  less  a  flow  of  even  ten  miles  largely 
purifies  the  water. 

Evidently  the  phenomenon  is  practically  identical  with  the 
bacterial  purification  of  sewage,  modified  by  the  different  conditions. 
The  following  factors  have  been  advanced  as  explaining  it: 

The  dilution  of  the  water  by  tributary  streams.  This  doubtless 
accounts,  in  part,  for  the  decrease  in  number  of  bacteria  per  c.c, 
but  it  cannot  be  a  very  important  factor  in  cases  such  as  shown. 


136  BACTERIA    IN    WATER. 

where  the  number  of  bacteria  in  the  river  finally  becomes  no  greater 
than  the  number  in  the  tributary  streams. 

The  action  of  sunlight  is  known  to  be  injurious  to  bacteria,  and 
it  has  been  thought  that  this  may  be  one  of  the  factors  destroying 
the  bacteria  in  streams.  But  its  action  in  muddy  streams  must  be 
very  slight. 

Other  living  organisms  in  the  water  have  a  deleterious  action. 
Microscopic  animals  certainly  destroy  great  quantities  of  bacteria, 
actually  feeding  upon  them,  and  they  may  be  one  of  the  efficient 
means  of  the  self-purification  of  streams. 

It  is  well  known  that  bacteria  are  generally  heavier  than  water 
and  that  they  will  slowly  sink  to  the  bottom.  In  slowly  flowing 
streams  sedimentation  probably  plays  an  important  part. 

The  food  in  the  water  is  of  course  used  up  either  by  bacteria  or 
some  other  organisms,  and  finally  becomes  insufficient  to  support 
bacteria  life. 

Although  these  factors  do  not  wholly  explain  the  purification 
of  streams,  it  is  certain  that  sewage-polluted  streams  are  in  time 
freed  from  most  of  their  bacteria.  Commonly,  however,  such  streams 
continue  to  receive  contamination  all  the  way  to  their  mouth,  and 
never  again  become  fit  for  drinking  purposes,  unless  the  water  is 
subsequently  purified  by  filtering  or  otherwise. 


PART  III. 

BACTERIA  IN  DAIRY  PRODUCTS. 


CHAPTER  XI. 

BACTERIA  IN  MILK. 

In  no  phase  of  farm  life  has  bacteriology  made  such  profound 
changes  as  in  dairy  methods,  changes  so  great  as  to  amount  almost 
to  a  revolution.  Many  dairy  methods  of  twenty-five  years  ago 
have  been  abandoned  and  many  new  ones  adopted,  chiefly  through 
the  discoveries  of  bacteriologists. 

BACTERIA  IN  MILK  WHEN  SECRETED. 

Milk,  when  secreted  from  the  mammary  gland  of  a  healthy  cow, 
is  generally,  if  not  always,  free  from  bacteria.  It  has  been  no  easy 
matter  to  demonstrate  this  fact,  since  there  are  bacteria  in  the  milk 
before  it  leaves  the  udder.  But  a  sufficient  number  of  careful  ex- 
periments have  shown  that  these  really  come  from  the  outside, 
entering  the  udder  through  the  milk  ducts,  and  that  they  do  not 
come  from  the  milk  glands. 

If  the  cow  is  not  in  perfect  health  her  milk  may  not  be  free  from 
bacteria.  When  a  cow  is  suffering  from  generalized  tuberculosis, 
or  when  she  has  this  disease  localized  in  the  udder,  her  milk,  when 
secreted,  is  sure  to  contain  bacteria.  Indeed,  any  udder  infection 
due  to  bacteria,  even  a  simple  inflammation  of  the  mammary  gland, 
is  likely  to  result  in  the  contamination  of  the  milk  with  the  bacteria 
which  cause  the  trouble.  Milk  from  a  cow  suffering  from  udder 
trouble  is  no  longer  pure  milk.  It  may  contain  tuberculosis  bacilli, 
12  137 


138  BACTERIA    IN    MILK. 

or,  in  cases  of  inflamed  udders,  it  is  likely  to  contain  pus^  together 
with  considerable  quantities  of  chain-forming  streptococci.  These 
should  not  be  present  in  good  milk,  and  there  is  reason  for  believing 
that  they  are  the  cause  of  certain  illnesses  in  man. 

Confining  our  attention  for  the  present  to  milk  from  healthy 
animals,  we  notice  that,  if  we  could  keep  bacteria  out  of  the  milk, 
none  of  the  ordinary  changes,  not  even  the  souring  which  is  so 
nearly  universal  in  normal  milk,  would  take  place.  Indeed,  milk 
which  is  free  from  bacteria  will  remain  visibly  unchanged  for  an 
indefinite  time.  It  is  not,  however,  absolutely  free  from  subsequent 
chemical  changes,  since  there  is  present  in  the  milk  an  enzyme  which 
produces  slow  changes.  This  enzyme,  called  galactase,  is  secreted 
by  the  milk  gland  with  the  milk,  and  may  thus  be  said  to  be  part  of 
the  milk.  It  can  slowly  convert  the  casein  of  the  milk  into  soluble 
proteids.  Its  action  is  very  slow,  however,  and  seemingly  of  no 
significance,  except  in  the  ripening  of  cheese.  At  all  events,  none 
of  the  ordinary  fermentations  appearing  in  milk  are  attributed  to 
this  galactase  or  to  any  other  part  of  the  milk  itself,  but  are  all  due 
to  microorganisms.  We  may,  therefore,  take  as  a  starting-point 
these  two  highly  important  facts:  i.  Milk  from  healthy  cows  will, 
if  it  could  be  kept  free  from  bacteria,  show  none  of  the  ordinary 
milk  fermentations.  2.  All  of  these  fermentations  are  due  to 
microorganisms  that  get  into  the  milk  after  the  milk  is  secreted  from 
the  mammary  gland. 

SOURCES  OF  MILK  BACTERIA. 

Recognizing  that  milk  is  germ  free  when  secreted  from  the  milk 
gland,  we  are  hardly  prepared  to  learn  that,  by  the  time  it  has  been 
drawn  from  the  cow,  received  in  the  milk-pail,  and  removed  from  the 
cow  stall,  it  may  contain  bacteria  to  the  extent  of  many  thousands 
per  c.c.  But  this  is  frequently  and,  indeed,  commonly  the  case. 
The  number  of  bacteria  in  freshly  drawn  milk  varies  greatly  with 
the  conditions  existing  in  the  dairy.  There  may  be  only  a  few 
hundreds  in  each  c.c,  or,  under  exceptional  conditions,  a  smaller 
number  still;  but  it  is  much  more  likely  that  the  milk,  by  the  time 


SOURCES  OF  MILK  BACTERIA.  139 

it  has  been  removed  from  the  stall,  contains  many  thousands  of 
bacteria. 

Since  all  the  troublesome  changes  which  occur  in  milk  and 
make  it  such  a  difficult  product  to  handle,  are  due  to  the  action  of 
bacteria  upon  the  milk,  it  is  to  the  interest  of  the  dairyman,  the  milk 
distributor,  and  the  consumer  to  have  as  few  bacteria  as  possible. 
Therefore,  it  is  a  matter  of  much  importance  to  learn  the  sources 
from  which  these  milk  bacteria  are  derived.  Knowledge  upon  this 
point  will  enable  the  dairyman  to  adopt  precautions  in  the  produc- 
tion and  caring  for  the  milk  that  will  materially  reduce  their  number. 
A  slight  attention  given  at  the  right  point  will  produce  better  results 
than  a  much  greater  attention  unintelligently  applied. 

The  Cow. — The  first  source  of  milk  bacteria  is  the  cow.  Al- 
though the  healthy  cow  secretes  milk  in  a  sterile  condition,  it  is  by  no 
means  sterile  when  it  leaves  the  milk  duct.  There  are  always  some 
bacteria  in  the  ducts  ready  to  be  washed  into  the  milking  pail  with  the 
first  jet  of  milk.  At  the  close  of  the  milking  enough  milk  is  left  in 
the  ducts  to  furnish  food  for  bacteria,  which  may  get  in  through  the 
external  opening ;  and  between  the  milkings,  at  the  warm  tempera- 
ture of  the  cow's  body,  these  bacteria  multiply  (Fig.  29).  Bacteria 
are  thus  always  abundant  near  the  opening  of  the  teat,  although 
the  inner  parts  of  the  duct  contain  smaller  numbers.  They  are  sure 
to  contaminate  the  first  jets  of  milk  drawn,  so  that  this  first  lot, 
called  fore  milk,  always  contains  more  bacteria  than  that  drawn 
later  in  the  milking.  Toward  the  close  of  the  milking  the  bacteria 
sometimes  disappear,  so  that  the  last  milk  may  be  actually  sterile 
when  it  leaves  the  duct.  While  this  is  not  always  the  case,  the  last 
milk  is  always  purer  than  the  first. 

From  these  facts  it  follows  that  milk  is  sure  to  contain  bacteria 
by  the  time  it  reaches  the  mouth  of  the  milk  duct.  While,  by  the 
use  of  special  precautions,  small  amounts  of  milk  can  be  drawn  so 
carefully  as  to  avoid  all  bacteria,  this  is  an  impossible  procedure  in 
dairying,  and  the  dairyman  must  recognize  that  there  is  no  practical 
means  by  which  he  can  obtain  sterile  miik.  Indeed,  it  would  avail 
but  little  if  he  could,  for  it  would  be  contaminated  almost  at  once 
from  other  sources. 


I40 


BACTERIA    IN    MILK. 


The  exterior  of  the  cow  is  an  even  more  prohfic  source  than  the 
milk  ducts.  Her  skin,  even  when  kept  in  fairly  good  condition,  is 
never  very  clean,  and  will  always  hold  more  or  less  dirt  and  dust 
laden  with  bacteria.     The  cow  in  many  poorly  kept  dairies  is  rarely, 


Fig.  29, — A  cow's  udder  cut  across  and  showing  the  milk  ducts. 


if  ever,  cleaned;  her  flanks,  tail,  and  skin  become  covered  with  a  coat- 
ing of  manure,  until  the  amount  of  filth  thus  attached  to  the  animal  is 
surprisingly  great.  All  of  this  filth  is  laden  with  bacteria,  and 
during  the  milking  process  numerous  particles  of  it  are  constantly 


SOURCES  OF  MILK  BACTERIA.  14I 

shed  from  her  body  by  the  movements  of  her  flanks,  by  the  switching 
of  her  tail,  and  by  the  rubbing  of  her  skin  by  the  milker.  Since  the 
milk-pails  are  generally  widely  opened,  they  receive  a  large  amount 
of  this  filth,  which  consists  of  almost  every  conceivable  kind  of 
material.  Besides  excrement  there  are  insect  wings,  grass,  straws, 
hairs,  and  many  other  small  particles,  all  bacteria  laden. 

Milk-vessels. — The  next  prolific  sources  of  bacteria  are  the  milk- 
pails  and  other  dairy  vessels,  in  which  the  bacteria  remain  alive 
from  one  milking-time  to  the  next.  On  an  ordinary  farm  these 
vessels  are  rarely,  if  ever,  washed  bacteriologically  clean;  for  washing 
in  hot  water  with  subsequent  drying  in  the  sun  is  wholly  insufficient 
to  remove  the  bacteria.  They  are  sure  to  remain  in  the  vessels, 
clinging  in  corners  and  cracks,  partly  dried  perhaps,  but  alive  and 
ready  to  begin  active  life  just  as  soon  as  they  are  supplied  with  the 
food  which  comes  to  them  in  the  next  lot  of  milk  drawn.  The 
ordinary  farm  has  no  really  effective  means  of  washing  milk- vessels. 
Even  live  steam,  as  ordinarilly  used,  a  few  seconds  on  each  pail,  will 
not  do  it  completely.  Many  a  troublesome  experience  of  the  milk 
dealer  in  warm  weather  is  attributable  directly  to  imperfectly 
washed  milk  cans,  and  disappears  at  once  when  all  the  milk-vessels 
are  thoroughly  sterilized  by  live  steam.  So  far  as  numbers  are  con- 
cerned, those  in  the  milk- vessels  probably  form  the  largest  source  of 
bacterial  contamination. 

The  Air. — Other  sources  furnish  bacteria  to  a  less  extent.  Some 
doubtless  come  from  the  air.  In  earlier  years  it  was  thought  that 
this  was  a  great  source  of  contamination,  but  now  we  know  that 
the  air  bacteria  are  ordinarily  of  little  importance,  although  some- 
times they  may  be  a  source  of  trouble.  Fresh,  out-of-door  air  does 
not  contain  many  bacteria,  and  if  milking  could  take  place  in  the  open 
air,  this  source  of  contamination  would  be  almost  excluded.  In  a 
close  barn,  however,  conditions  are  very  different.  The  motions  of 
the  crowded  cattle  dislodge  bacteria  from  their  skins.  Hay,  dirt, 
cobwebs,  soiled  bedding  and  other  dry  dust-producing  materials 
are  allowed  to  accumulate,  and  particles  from  any  of  these  sources 
are  likely  to  be  dislodged  and  float  for  a  time  in  the  air.  The  gen- 
eral manner  of  feeding  the  animals  is  even  a  larger  sourcTe  of  contam- 


142  BACTERIA    IN    MILK. 

ination.  If  dry  hay  or  other  dry  food  is  thrown  down  in  front  of  the 
cattle,  a  large  amount  of  dust  will  arise  and  spread  through  the  air 
of  the  stable.  Such  dust  is  crowded  with  bacteria,  many  of  which 
are  alive  and  will  settle  into  the  milk-pail  during  milking.  The 
common  practice  of  keeping  cattle  in  the  same  room  where  they 
are  milked  is  thus  very  productive  of  a  large  source  of  bacterial 
contamination. 

The  Milker. — Of  late  years  it  has  become  evident  that  the 
bacteria  coming  from  the  milker  or  other  persons  in  the  dairy  are 
among  the  most  serious.  This  is  not  so  much  because  of  the 
number  of  bacteria  that  may  enter  the  milk  from  this  source,  but 
because  of  their  types.  In  ordinary  dairies  the  milker  rarely  makes 
any  special  toilet  before  milking,  but  is  liable  to  perform  this  task  in 
old,  soiled  clothing,  with  no  attempt  at  cleaning  his  hands  and  face. 
Under  these  circumstances,  while,  so  far  as  concerns  numbers,  he  is 
not  so  great  a  source  of  bacteria  as  the  cow,  some  of  these  organisms 
are  sure  to  fall  from  his  hands  or  clothes  into  the  milk-vessels, 
especially  if  he  adopts  the  filthy  habit  of  wet  milking.  The  number 
of  bacteria  from  such  a  source  is,  probably,  not  great,  and  does  not 
add  materially  to  the  bacterial  content.  But  in  one  respect  these 
bacteria  assume  a  more  important  significance.  The  bacteria 
which  produce  diseases  in  one  animal  do  not  necessarily  produce 
diseases  in  other  animals.  Those  which  produce  diseases  in  cattle, 
with  some  exceptions  (tuberculosis),  do  not  usually  have  the  same 
effect  on  man.  But  it  is  evident  that  any  disease  germs  that  may  be 
present  in  one  man  are  just  the  kind  that  can  develop  in  any  other 
human  being.  Therefore,  bacteria  contamination  from  human 
sources  is  more  dangerous  to  other  human  beings  than  any  infection 
from  animals.  For  this  reason  the  bacteria  which  enter  the  milk 
from  the  milker  are  liable  to  be  more  dangerous  than  those  which 
come  from  any  other  source. 

TYPES  OF  BACTERIA  FOUND  IN  MILK. 

Many  different  types  of  bacteria  get  into  milk  from  these  various 
sources.     Some  of  them  are  useful,  some  are  of  no  particular  signifi- 


TYPES  OF  BACTERIA  FOUND  IN  MILK.  143 

cance,  some  are  troublesome  to  the  dairyman  though  not  distinctly 
harmful,  while  some  are  decidedly  injurious  either  to  the  dairy  prod- 
ucts or  to  man.  A  knowledge  of  these  types  is  of  primal  impor- 
tance to  an  understanding  of  their  relations  to  dairying.  The  more 
important  types  are  given  in  the  following  pages.  For  clearness  and 
convenience  we  may  divide  them  into  three  groups:  i.  Normal 
milk  bacteria.  2.  Abnormal  milk  bacteria.  3.  Disease  bacteria.  The 
first  two  concern  dairy  problems  only,  while  the  last  concerns  the 
relation  of  milk  to  the  public  health. 

I.    NORMAL   MILK   BACTERL\. 

Under  this  head  we  refer  to  types  of  organisms  that  are  practically 
always  present  in  milk  and  cannot  be  avoided  by  any  ordinary 
means.  They  do  not,  of  course,  belong  to  the  milk,  but  they  are  so 
widely  distributed  in  barns  and  dairies  that  practically  they  cannot 
be  avoided.  There  are  very  many  different  kinds  among  them, 
several  scores  at  least  having  been  described  in  milk  from  various 
localities.  But  they  may  be  conveniently  grouped  and  studied 
under  three  heads: 

Lactic  Acid  Bacteria. — The  most  common  fermentation  of 
milk  is  its  souring,  a  phenomenon  so  universal  that  it  has  been  sup- 
posed to  be  a  change  belonging  to  milk  itself.  But  it  is  now  known 
to  be  produced  always  by  the  growth  of  bacteria.  These  organisms 
transform  the  milk  sugar  into  lactic  acid,  a  change  that  is  sometimes 
expressed   by  the   formula  CgHj^Og  =  2031^603;  but  this  equation 

(Sugar)  (Lactic  acid) 

fails  to  express  the  real  nature  of  the  change  that  occurs,  which  is 
much  more  complex.  The  fundamental  phenomenon,  however,  is 
that  the  milk  is  made  sour  by  the  formation  of  lactic  acid  out  of  milk 
sugar.  This  is  first  seen  in  the  appearance  of  a  sour  taste  and  later 
in  the  curdling.  Milk  contains  its  casein  in  a  state  of  partial  solu- 
tion, but  if  the  milk  is  made  sufficiently  acid  the  casein  can  no  longer 
remain  in  solution  and  is  precipitated.  The  precipitation  of  casein 
is  the  curdling  of  milk,  and  it  occurs  when  we  add  to  it  any  kind  of 
acid.  In  the  normal  souring  of  milk,  when  the  acid  reaches  0.7  per 
cent,  to  0.9  per  cent,  the  milk  curdles. 


144  BACTERIA    IN    MILK. 

The  Souring  of  Milk  during  Thunder  Storms. — The  only  natural 
agent  that  causes  souring  is  the  growth  of  microorganisms.  There 
is,  however,  a  wide-spread  belief  that  thunder  storms  will  sour  and 
curdle  milk.  This  belief  rests  upon  a  mistaken  interpretation  of 
observed  facts.  It  is  certainly  true  that  milk  is  frequently  found 
sour  after  a  thunder  storm,  and  the  natural  interpretation  is  that 
the  electricity  of  the  storm  has  produced  the  souring.  A  careful 
study  of  the  phenomenon  has  shown  that  this  inference  is  incorrect. 
Electricity,  in  the  form  of  a  current  or  electric  sparks,  has  no  power 
to  sour  milk;  and,  further,  if  milk  is  kept  properly  cooled,  the  thunder 
storm  has  no  effect  upon  it.  Moreover,  if  milk  has  been  deprived 
of  bacteria,  it  will  keep  indefinitely,  remaining  sweet  in  spite  of 
thunder  storms.  In  short,  all  evidence  shows  that  the  thunder 
storm  has  no  power  of  souring  milk,  unless  bacteria  are  present  to 
produce  the  lactic  acid,  and  that  thunder  and  lightning  have  no 
direct  effect  upon  the  souring  of  milk. 

What,  then,  brings  about  the  frequent  souring  of  milk  during 
thunder  storms  and  the  wide-spread  belief  that  thunder  is  the  cause  ? 
The  answer  seems  to  be  the  simple  one,  that  the  same  agencies 
which  produce  the  thunder  storm  cause  a  rapid  growth  of  bacteria. 
The  thunder  storm  is  brought  on  by  climatic  conditions,  dependent 
chiefly  upon  the  temperature,  and  these  same  conditions  are  just 
those  that  stimulate  bacterial  growth.  It  will  thus  happen  that  the 
same  sort  of  warm  weather  which  produces  the  thunder  storm  also 
hastens  the  growth  of  bacteria  in  milk  if  not  kept  artificially  cooled 
with  ice.  It  will  frequently  happen,  as  a  result,  that  the  milk  will 
be  ready  to  show  signs  of  souring  at  the  same  time  that  the  thunder 
storm  appears,  frequently  in  the  afternoon.  The  two  phenomena 
occur  together,  not  because  the  one  causes  the  other,  but  because 
the  same  climatic  conditions  which  produce  the  storm  hasten  the 
growth  of  bacteria.  A  similar  warm  spell  will  sour  the  milk  just 
as  quickly,  even  though  no  thunder  storm  appears.  Whether  this 
is  the  whole  explanation  may  be  doubtful,  but  it  is  clearly  demon- 
strated that  the  thunder  and  lightning  have  nothing  to  do  directly 
with  the  phenomenon.  The  souring  of  milk  is  always  produced  by 
bacteria. 


TYPES    OF    BACTERIA    FOUND    IN    MILK.  1 45 

Varieties  of  Lactic  Acid  Bacteria. — A  very  large  number  of  ap- 
parently different  kinds  of  acid-forming  bacteria  have  been  obtained 
from  milk.  The  different  varieties  all  agree  in  producing  lactic 
acid,  but  differ  in  some  other  slight  points,  recognized  by  bacteri- 
ologists. To  what  extent  these  many  varieties  should  be  combined 
so  as  to  make  a  small  number  of  groups,  and  to  what  extent  they 
should  be  kept  separate,  is  a  matter  over  which  there  is  as  yet  no 
agreement.  It  is  known  that  the  same  bacterium  can  show  differ- 
ences under  different  conditions.  The  power  of  a  bacterium  to 
curdle  milk  may  be  increased  by  proper  laboratory  methods, 
and  when  we  find  that,  of  these  numerous  described  types,  some 
differ  from  others  only  in  the  rapidity  with  which  they  curdle  milk, 
we  naturally  infer  that  the  different  results  are  brought  about  by 
the  same  bacterium  growing  under  slightly  different  conditions. 
Those  who  have  given  the  most  attention  to  the  subject  are  convinced 
that  the  lactic  acid-forming  bacteria  that  have  been  described  must 
be  reduced  to  a  few  types,  though  no  one  yet  ventures  to  say  how 
few. 

Among  them  are  three  well-marked  types,  quite  radically  distinct 
from  each  other,  and  each  playing  an  important  part  in  the  dairy. 
Two  of  them  are  the  dairyman's  friends,  while  the 
other  is  always  his  foe.'  'S^g^<^ 

I.  Bacterium  acidi  lactici,  Streptococcus  lacticus. —       q^^^ 
These  two  names  are  applied  to  the  same  organism. 
The  first  name  was  originally  given  to  it  when  it  was     ^  ^^^/   ^o-— 

o  -^   o  Bacterium 

described  as  a  short  rod  (Fig.  30).  Recently  it  has  acidi  lac t id 
been  claimed  that  it  is  not  a  rod,  but  a  coccus,  and  lactkts)!'^^^^ 
with  this  conception  the  second  name  has  been  given  as 
the  only  correct  one.  Which  of  these  two  names  is  more  correctly 
applied  has  not  yet  been  settled.  But  whatever  its  name  and 
microscopic  appearance,  it  is  a  quite  well-known  organism,  with  a 
distinctive  action  on  milk.  This  type  of  lactic  acid  bacterium 
grows  better  when  not  in  free  contact  with  the  air.  It  grows  better 
under  the  surfaces  of  media  than  on  the  surface,  failing  to  make 
any  visible  growth  on  the  surface  of  potato  and  scarcely  any  on 
agar  culture  slants  (see  page  313).  In  milk,  however,  it  grows 
13 


146  BACTERIA    IN    MILK. 

with  great  rapidity,  soon  turning  it  acid.  The  rapidity  of  the  acid 
production  is  variable  with  different  cultures.  It  is  so  rapid  in 
some  cases  that,  if  the  specimen  be  placed  at  body  heat,  it  will 
curdle  in  six  hours.  With  other  cultures,  the  curdling  under  similar 
circumstances  would  not  occur  for  three  days;  with  some  cultures 
curdling  never  occurs.  All  specimens  of  milk,  however,  become 
acid,  although  not  always  sufficiently  to  precipitate  the  casein. 
Between  these  extremes  every  conceivable  grade  may  be  found  in 
cultures  that  are,  in  other  respects,  identical;  and  they  represent, 
doubtless,  one  type,  differing  in  its  power  of  producing  acid. 

To  this  type  belongs  the  largest  number  of  bacteria  known  to 
cause  the  souring  of  milk.  Most  of  the  butter  starters  and  cheese 
starters  (see  page  189)  belong  to  this  general  class.  But  the  name 
represents  a  type  rather  than  any  single  organism.  In  other  words, 
B.  acidi  lactici  represents  a  group  of  closely  allied  varieties.  If  we 
are  asked  whether  it  represents  a  species  or  a  collection  of  species, 
we  must  answer  that  no  one  knows  what  is  meant  by  the  term 
species  among  bacteria.  The  term  species,  whatever  its  significance 
among  higher  animals  and  plants,  seems  to  have  no  meaning  among 
bacteria.  It  is  impossible,  therefore,  to  say  whether  Bad.  lactis 
acidi  is  a  single  species  or  a  group  of  species;  and  we  may  be  content 
simply  to  recognize  under  this  name  the  group  of  lactic  acid  bacteria 
which  most  commonly  cause  milk  souring  and  which  comprise 
varieties  that,  while  agreeing  in  most  respects,  have  slightly  differing 
characters. 

The  type  of  milk  curdling  produced  by  this  organism  is  quite 
easily  recognized.  The  milk  becomes  strongly  acid,  and  turns 
into  a  hard  curd,  without  any  trace  of  gas  bubbles,  and  without  the 
separation  of  whey:  it  has  a  clean,  sharp  taste,  and  no  odor  (Fig. 
31,  h).  This  type  of  curdling  has  been  recognized  as  a  desirable 
one  by  the  dairyman,  since  it  is  most  favorable  for  dairy  processes 
and  is  consistent  with  the  production  of  the  best  grades  of  butter 
and  cheese.  This  organism  grows  readily  at  temperatures  from 
60°  to  100°  F.,  growing  more  rapidly  at  higher  temperatures.  At 
a  temperature  of  about  70°  it  grows  with  great  rapidity,  and  at 
this  temperature  it  seems  to  be  more  vigorous  than  any  other 


TYPES    OF    BACTERIA    FOUND    IN    MILK. 


147 


bacterium  ordinarily  found  in  milk.  For  this  reason,  as  we  shall 
presently  see,  milk  kept  at  70°  becomes,  in  a  short  time,  almost 
completely  filled  with  this  species  of  bacterium  at  the  expense  of 
all  the  others  that  might  originally  have  been  there.  This  type  of 
organism  is  a  friend  to  the  dairyman,  unless  he  be  a  milk  man  who 
wants  to  deliver  his  milk  sweet. 


Fig.  31. — Showing  the  action  of  different  types  of  bacteria  upon  milk,  a,  the 
aerobic  type  {Bad.  aerogenes);  h,  the  anaerobic  type  {Bad.  acidi  lactici);  c  and  d,  the 
peptonizing  type,  with  the  curd  in  different  stages  of  digestion. 


II.  Bacterium  aerogenes. — This  type  of  lactic  acid  organism  is  not 
so  abundant  in  milk  as  the  first,  although  it  may  be  found  more  or 
less  abundant  in  most  samples  of  milk.  Microscopically,  it  is 
practically  indistinguishable  from  the  first  variety,  but  it  differs  in 
two  pronounced  points  (Fig.  32).  The  first  is  that  it  grows  luxuri- 
antly in  contact  with  the  air  or  oxygen,  while  the  first  variety  does 


148  BACTERIA    IN    MILK. 

not.  The  aerogenes  type,  therefore,  grows  abundantly  on  the  sur- 
face of  culture  media,  like  potato  or  agar.  The  second  and  more 
noticeable  point  is  the  fact  that  it  produces  a  fermentation  of  milk- 
sugar,  giving  rise  to  a  quantity  of  gas.  When  inoculated  into  milk 
it  causes  a  souring  which  is  rapidly  followed  by  curdling,  the  rapidity 
of  the  curdling  varying  in  different  specimens.  The  curd  which  is 
produced  differs  very  much  from  that  of  the  first  type 
of  lactic  acid  bacteria.  It  is  always  more  or  less  filled 
with  gas  bubbles,  and  when  care  is  taken  to  obtain  a 
typical  curd,  it  appears  crowded  with  holes,  which 
Fig.  32.—  represent  the  bubbles  of  gas  formed  by  the  organism 
genes.  *  ^'  (-^^S*  3^)'  '^^^  whey  commonly  separates  in  a  short 
time  from  the  curd,  and  the  final  appearance  is  strik- 
ingly different  from  that  of  the  curdled  milk  produced  by  the 
first  type. 

The  production  of  gas  is  the  cause  of  the  ruin  of  vast  quantities 
of  cheese.  If  milk,  when  it  is  made  into  cheese,  contains  a  consider- 
able quantity  of  these  bacteria,  instead  of  the  more  common  type, 
the  bacteria  grow  and  develop  gas,  the  cheese  becomes  filled  with 
the  bubbles,  swelling  more  and  more,  until  it  finally  results  in  what 
is  known  as  swelled  cheese  (Fig.  ^7,).  At  the  same  time  that  this 
swelling  occurs,  the  flavor  of  the  cheese  becomes  unsatisfactory,  so 
that  the  swelled  cheese  may  be  practically  worthless.  These  or- 
ganisms have  been  the  cause  of  the  loss  of  enormous  amounts  of 
money  to  cheese  makers.  In  butter-making  they  are  not  so  dis- 
astrous, but  here,  too,  their  presence  is  undesirable,  for  they  some- 
times produce  unpleasant  flavors  in  the  creams,  resulting  in  an 
inferior  grade  of  butter.  This  type  of  organism,  therefore,  is 
decidedly  the  dairyman's  foe. 

Several  varieties  of  bacteria  belong  to  this  general  type  of  gas- 
producing  organisms.  Among  them  is  B.  coli,  which  is  very 
similar  to  B.  aerogenes,  except  that  it  is  motile.  This,  an  inhabitant 
of  the  intestine  (see  page  130)/ is  very  commonly  found  in  milk. 

III.  Bacillus  Bulgarious. — A  third  radically  different  type  of  acid 
bacterium  is  one  that  has  recently  come  into  prominence  in  various 
forms  of  beverages  composed  of  soured  milk.     In  certain  parts  of 


TYPES    OF    BACTERIA   FOUND    IN    MILK.  149 

Europe  sour  milk  is  commonly  used  as  a  beverage,  and  has  been 
highly  recommended  as  a  healthful  drink.  It  is  claimed  that  the 
lactic  acid,  present  in  abundance  in  these  sour  milks,  has  a  very 
beneficial  action  in  the  intestine,  preventing  the  growth  of  the  com- 
mon putrefactive  germs,  and  serving  in  general  as  a  corrective  for 
various  intestinal  disturbances.     It  is  not  ordinary  sour  milk  that  is 


Fig.  33. — Curds  from  two  lots  of  milk  soured  by  a  gas-forming  and  a 
non-gas-forming  bacterium. 


especially  recommended  for  this  purpose,  but  a  special  form,  found 
most  common  in  Bulgaria,  and  used  veiy  widely  as  a  drink  in  that 
country.  Such  milk  contains  a  variety  of  lactic  acid  bacterium 
very  different  from  the  two  above  described,  and  deserving  to  be 
called  a  distinct  type.  It  is  in  the  form  of  a  long,  large  rod  (Fig.  34) , 
which  frequently  forms  long  chains.  It  differs  from  the  more  com- 
mon type  in  producing  a  much  larger  quantity  of  lactic  acid.     The 


150  BACTERIA    IN    MILK. 

ordinary  sour  milk  organisms  produce  from  1.2  per  cent,  to  1.5  per 
cent,  of  lactic  acid,  and  then  cease  to  grow;  but  this  Bulgarian  type 
produces  as  much  as  3.0  per  cent,  of  acid,  double  the  amount  pro- 
duced by  the  common  type.  This  type  is  very  vigorous  and  when 
growing  in  milk  will  soon  destroy  other  bacteria.  Quite  a  number 
of  commercial  products  containing  this  organism  are  now  on  the 
market,  and  are  used  somewhat  widely  in  making  a  fermented  milk. 

Though  originally  found  in  Bulgaria,  bacteria  that 

y^y^^^      agree  with  it  in  all   essential   respects   have  been 

X    /'—^\       found  elsewhere.     It  has  been  found  in  this  country 

as  well  as  in  Europe,  but  thus  far  on  grain  rather 
^garicus.     " "   than  in  milk.     Several  of  the  fermented  milks  found 

in  different  countries  appear  to  contain  representa- 
tions of  this  type  of  lactic  acid  organism. 

Peptonizing  and  Rennet-forming  Bacteria. — Occasionally 
a  dairyman  is  puzzled  by  a  somewhat  unusual  phenomenon:  his 
milk  curdles,  but  remains  sweet.  This  is  apt  to  occur  in  the  fall  or 
spring  when  the  food  of  the  cattle  is  being  changed,  and  is  due  to  a 
class  of  bacteria  that  secrete  enzymes.  The  bacteria  in  question 
really  secrete  two  enzymes,  one  of  which  is  similar  to  rennet^  secreted 
by  the  stomach  of  a  calf,  and  the  other  is  similar  to  trypsin^  se- 
creted by  the  pancreatic  gland  of  man  and  other  animals.  Hence 
these  bacteria  secrete  two  enzymes  that  have  actions  essentially 
like  those  of  digestvie  fluids. 

When  this  class  of  bacteria  grow  in  milk,  both  of  these  enzymes 
act. upon  it.  The  rennet  enzyme  shows  its  effect  first,  and  causes  the 
milk  to  curdle;  but  since  no  acid  is  produced  by  these  organisms  this 
curd  is  not  sour.  The  curd  is  also  softer  than  that  produced  by 
the  lactic  acid  bacteria.  The  phenomenon  is  sometimes  called 
s^eet  curdling.  After  a  short  time,  usually  two  or  more  days,  the 
second  enzyme  begins  to  show  its  effects.  This,  acting  like  a 
digestive  fluid,  changes  the  nature  of  the  casein  from  an  insoluble 
to  a  soluble  condition,  and  as  fast  as  this  occurs  the  curd  is  dis- 
solved in  the  liquid  of  the  milk.  The  curd  thus  disappears,  as  the 
casein  is  dissolved,  and,  finally,  the  whole  curd  may  be  dissolved  so 
that  the  milk  becomes  liquid  again  (Fig.  31,  c,  (/).    But  it  is  a  totally 


TYPES  OF  BACTERIA  FOUND  IN  MILK. 


151 


different  product  from  the  original  milk,  since  it  no  longer  contains 
casein,  but  only  the  digested  and  dissolved  products  of  casein^ 
This  softening  and  dissolving  of  the  curd  is  so  much  like  the  diges- 
tion that  goes  on  in  the  intestine  of  animals,  that  it  has  commonly 
been  called  digestion.  The  bacteria  that  produce  it  are  also  some- 
times called  the  peptonizing  bacteria,  since  they  produce  peptones 
among  the  soluble  products  that  come  from  the  digested  casein. 

Still  another  term  is  applied  to  these  bacteria.     One  of  the 
common  culture  media  used  in  bacteriological  work  is  solidified 


w  \J 


:f 


-£^ 


F;f? 


vJ  v_y  v_y  v_y 


Fig.  35. — Gelatin  stab  cultures.    e,f,  and  g,  show  liquefaction;  a,  filiform;  b,  beaded; 
c,  villous;  d,  arborescent;  e,  napiform;  /,  infundibuliform;  g,  stratiform. 


with  gelatin.  Now,  many  kinds  of  bacteria  cause  the  solidified  jelly 
to  become  liquid  (Fig.  35  E.  F.  G.).  The  liquefaction  of  the  gelatin 
is  due  to  the  same  enzyme  that  causes  the  digestion  of  the  milk  curd. 
Hence,  we  find  that  the  bacteria  that  liquefy  gelatin  commonly 
have  the  power  of  dissolving  milk  curd.  The  term  liquefiers  is 
applied  to  them,  since  they  liquefy  both  curdled  milk  and  gelatin. 
This  type  of  bacteria  is  very  abundant,  and  in  the  number  of  kinds 
exceeds  the  number  of  lactic  acid  bacteria.  They  are  found  pro- 
fusely in  the  dairy,  especially  in  the  filth  that  gets  into  the  milk. 
Practically  every  sample  of  fresh  milk  will  contain  them  in  greater 
or  less  numbers.  But  it  is  very  doubtful  whether  this  type  of  bacteria 
is  of  much  or  of  any  significance  in  ordinary  daiiying.  Although 
the  varieties  may  be  numerous  and  their  number  may  be  great  in 


152  BACTERIA    IN    MILK. 

fresh  milk,  they  very  rarely  get  an  opportunity  to  have  any  consider- 
.  able  effect  upon  the  milk.  The  lactic  bacteria  grow  so  very  much 
more  rapidly  that  they  soon  entirely  outnumber  the  enzyme  class, 
and,  indeed,  in  most  cases  stop  their  growth.  As  a  result,  whereas 
the  latter  may  be  comparatively  numerous  in  fresh  milk,  they  become 
less  rather  than  more  abundant  as  the  lactic  bacteria  grow,  and 
finally  disappear.  Under  such  conditions  their  significance  in  the 
milk  is  probably  nothing.  Occasionally,  however,  it  may  happen 
that  a  sample  of  milk  does  not  chance  to  have  any  lactic  organisms 
in  it,  or  that  they  are  so  few  as  to  fail  to  get  the  upper  hand  of  the 
others.  If  this  occurs,  the  other  species  of  bacteria  may  find  the 
conditions  favorable  to  their  growth,  as  in  cases  of  sweet  curdling. 
This  class  of  bacteria  plays  an  important  part  in  the  changes  which 
may  take  place  in  so-called  sterilized  milk,  which  has  been  heated 
to  a  temperature  of  boiling  water.  Such  milk  still  contains  a 
considerable  number  of  spore-bearing  bacteria  that  resist  this 
temperature.  The  milk  does  not  sour,  inasmuch  as  all  lactic  acid 
bacteria  are  killed,  since  they  never  produce  spores.  The  class  of 
enzyme-forming  bacteria,  however,  are  very  commonly  spore 
bearers,  and  resist  the  temperature  of  boiling  water.  Milk  which 
has  been  boiled,  therefore,  not  infrequently  undergoes  changes 
which  affect  its  taste  and  its  chemical  nature,  due  to  the  class  of 
bacteria  here  considered.  Occasionally  they  are  of  significance  in 
cheese-making.  During  the  long  ripening  of  cheese  they  have  a 
better  chance  to  grow  than  in  milk.  Whether  they  have  much 
influence  upon  hard  cheeses  seems  doubtful,  but  in  the  ripening  of 
soft  cheeses  they  sometimes  produce  very  bad  results,  causing  much 
loss  to  the  cheese-makers.  While,  therefore,  they  are  of  little  impor- 
tance to  the  one  who  handles  milk,  they  play  a  considerable  part 
in  the  making  of  cheese. 

This  class  of  liquefying  bacteria  usually  produces  no  acid; 
but  there  is  a  small  group  of  the  same  class  that  differs  from  the 
others  in  producing  both  a  digesting  enzyme  and  an  acid.  They 
are  sometimes  called  acid  liquefiers.  It  has  been  thought  that  they 
play  a  part  in  the  ripening  of  cheese,  but  this  is  by  no  means 
certain  and  in  general  they  are  of  little  significance. 


TYPES    OF    BACTERIA    FOUND    IN    MILK.  1 53 

Neutral  Types. — Among  the  normal  milk  bacteria  there  are 
many  that  appear  to  have  no  noticeable  action  on  milk.  They 
produce  no  acid  and,  apparently,  no  enzyme.  When  they  grow 
in  milk  they  produce  no  noticeable  effect  upon  it.  So  far  as  we 
can  see  they  are  of  no  significance  in  dairying.  Nor  do  we  have 
any  reason  for  believing  that  they  have  any  pathogenic  effect  upon 
persons  drinking  the  milk.  They  are,  therefore,  simply  classed  as 
neutral  types,  and  need  not  here  be  further  considered. 

ABNORMAL   MILK   BACTERIA. 

The  types  of  milk  bacteria  included  under  this  head  differ 
from  those  already  considered  merely  in  the  fact  that  they  are 
comparatively  rare.  Whereas  milk  will  practically  always  sour 
through  the  agency  of  the  lactic  bacteria  and  will  nearly  always 
contain  bacteria  of  the  peptonizing  class,  the  following  kinds  of 
bacteria  are  not  commonly  found.  Most  of  them  are  occasionally 
the  cause  of  troublesome  dairy  infections.  When  they  occur  in 
milk,  in  numbers  sufficient  to  cause  troublesome  changes,  they 
may  always  be  regarded  as  coming  from  some  unusual  source  of 
contamination,  one  which  may  be  prevented.  While  souring  of 
milk  cannot  be  prevented  by  any  practical  means,  because  of  the 
universal  distribution  of  lactic  acid  bacteria,  these  types  of 
troublesome  infections  may  be  prevented  if  sufficient  care  is  taken 
in  regard  to  cleanliness,  and  they  may  be  checked  if  the  dairy- 
man simply  learns  whence  the  contamination  arises.  For  these 
reasons,  in  practical  dairying,  it  is  a  matter  of  special  importance 
to  understand  their  sources. 

Slimy  Milk. — Slimy  milk  is  not  an  uncommon  trouble  in  the 
dairy.  It  is  sometimes  produced  by  a  diseased  condition  of  the 
cow,  slimy  milk  being  a  common  characteristic  of  garget.  In 
such  cases  the  milk  is  slimy  when  drawn.  Such  milk  is  certainly 
not  fit  to  drink. 

In  other  cases  the  milk  is  not  slimy  when  drawn,  but  appears 
like  normal  milk.  *  After  a  few  hours,  at  about  the  time  when 
milk  would  usually  sour,  instead  of  becoming  acid  in  the  normal 


154  BACTERIA    IN    MILK. 

way,  it  becomes  viscid,  and  finally  it  may  be  so  slimy  that  it  can 

be  drawn  out  into  long  threads.     At  the  same  time  it  has  a  sweetish 

taste.     Such  milk  is  practically  worthless.     It  cannot  be  used  for 

butter-making,  for  the  cream  will  not  separate.     It  will  not  be  used 

for  drinking  oi  cooking  purposes,  although  there  seems  to  be  no 

reason  for  believing  that  it  is  not  perfectly  wholesome.     In  some 

countries,  indeed,  such  slimy  milk  is  a  favorite  beverage;  but  in  this 

^^  country  most  people,  not  wishing  to  drink  slime,  will 

.-^?#  throw  it  away.     Sometimes  such  an  infection  proves 

'K%  very  troublesome.      It  may  spread  through  a  whole 

^#1         farming  district,  affecting  many  dairies  and  continuing 

^OV  \  *#       fo^  ^  lo^g  time.     Although  not  always  easy  to  follow, 

•  ft*  such  infections  may  generally  be  traced  to  some  com- 

F1G.36.— s.     mon  source  of  distribution.     For  example,  a  central 

lactisvisCOSUs;  .     .  it"  ^^^      c 

the  common  Creamery,  receivmg  such  slimy  milk  from  some  patron, 
^^Mrrw/'^'Jf  may  distribute  the  trouble  over  the  whole  patronizing 
district  by  returning  to  the  farmers  the  milk  vessels 
not  properly  sterilized. 

The  cause  of  this  sliminess  is  the  growth  of  bacteria.  Several 
different  kinds  of  bacteria  have  been  discovered  with  this  property. 
The  best  known  of  them,  and  probably  the  most  common,  is  one  that 
has  been  named  B.  lactis  viscosus  (Fig.  36).  This  has  been  found 
to  be  the  cause  of  the  trouble  in  Europe,  and  a  similar  if  not  the 
identical  organism  has  been  found  in  America.  It  appears  to  be 
a  very  vigorous  organism,  and,  when  once  present,  will  grow  so 
rapidly  as  to  make  the  milk  slimy  in  spite  of  the  action  of  the  ordinary 
acid-forming  bacteria  that  may  be  present. 

To  understand  the  sources  from  which  this  troublesome  organism 
is  derived  may  be  a  matter  of  great  importance  to  a  dairyman. 
Three  sources  have  thus  far  been  detected:  i.  Sometimes  it  may 
come  from  water  used  in  washing  the  milk  cans  or,  more  likely, 
from  the  water  in  which  the  cans  have  been  standing  to  cool  the 
milk.  2.  It  may  come  from  the  udder  of  the  cow;  perhaps  a  single 
cow  in  a  herd  being  thus  infected  and  her  milk  contaminating  that 
of  the  whole  dairy.  3.  Slimy  milk  bacteria  have  been  found  in  the 
dust  of  the  air  in  dairies.     If,  therefore,  a  dairy  is  troubled  with 


TYPES    OF    BACTERIA    FOUND    IN    MILK.  1 55 

slimy  milk,  the  dairyman  should  look  first  to  his  water-supply,  es- 
pecially if  the  milk  has  been  cooled  by  standing  in  cans  in  the  water. 
Then  he  may  turn  his  attention  to  the  food  of  the  cows  to  see  if  he 
has  any  special  lot  of  hay  or  other  food  that  holds  the  troublesome 
organisms.  This  may  be  tested  by  changing  the  food  for  a  time. 
Lastly,  he  will  do  well  to  keep  the  milk  of  the  different  cows  separate 
for  a  few  days,  to  see  if  the  trouble  can  be  traced  to  any  particular 
cow.  Having  once  found  the  source,  the  remedy  is  simple:  either 
by  applying  some  method  of  disinfection  at  the  source  of  infection, 
or  seeing  that  infected  water  does  not  come  in  contact  with  the  milk 
cans,  or  removing  the  milk  of  the  cow  that  is  at  fault,  or  changing 
the  food. 

Bitter  Milk. — Next  to  slimy  milk,  perhaps  bitter  milk  offers  the 
most  trouble  to  the  dairyman.     Three  quite  different  sources  of 
bitter   milk  can   be  distinguished:      i.    The  cow.     She   may  give 
bitter  milk  because  of  improper  food,  such  as  lupines,  which  will 
impart  a  bitter  taste.     Bitter  milk  is  also  quite  common  in  a  late 
stage  of  lactation.     These  types  may  be  recognized  by  the  fact  that 
the  milk  is  bitter  as  soon  as  it  is  drawn  from  the  cow,  and  the  bitter- 
ness does  not  increase  later.     2.  Microorganisms.       '  ^ 
In  such  cases  the  bitterness  is  a  matter  of  slow          •^   A^Vx 
development.     The  milk,  when  drawn,  tastes  as        Hff      ^   (S 
usual,  and  the  bitterness  appears  after  standing  a       ^         (J  ^ 
few  hours,  increasing  in  intensity  untiL,  in  a  short        Fig.  37.— Organ- 
time,  it  is  at  its  maximum.     In  these  instances     |sms  producing  bitter 

'  milk,     a,  A  coccus; 

the  bitterness  is  produced  by  microorganisms  b,  a  yeast  Toruia 
which  grow  in  the  milk.    Two  or  three  varieties  of  ^      amson  . 

bacteria  have  been  described  that  have  this  power,  and  have  been 
the  cause  of  a  troublesome  bitter  fermentation  in  milk  (Fig.  37,  a). 
The  source  of  the  trouble  has  been  traced,  in  one  case,  to  organisms 
in  the  udders  of  a  single  cow  in  a  herd.  Bitter  milk  is  not  of  very 
common  occurrence.  In  cheeses  the  development  of  a  bitter  taste 
is  much  more  common,  doubtless  because,  during  the  ripening,  the 
bacteria  have  a  longer  time  to  develop  their  bitter  products.  A 
bitter  taste  in  cheese  has  been  in  some  cases  traced  to  bacteria,  and 
in  one  extended  series  of  troubles,  which  affected  the  cheese-making 


156  BACTERIA    IN    MILK. 

over  an  extensive  territory,  the  cause  was  found  to  be  due  to  a  yeast 
that  infected  the  factory  and  the  utensils  used  in  cheese-making 
(Fig.  37,  6).  The  remedy  in  such  cases  lies  in  a  thorough  disinfec- 
tion of  the  cans,  vats,  etc.  3.  Microorganisms  in  boiled  milk.  It 
may  frequently  happen  that  boiled  milk  will  become  bitter.  The 
boiling  destroys  the  acid-forming  bacteria,  but  leaves  alive  some  of 
the  spore-producing  organisms.  These  may  subsequently  develop 
and  produce  bitter  products.  This  type  of  bitter  milk  is  of  little 
significance,  however,  since  the  practice  of  keeping  milk  after  it  is 
boiled  has  almost  disappeaied. 

Fermentations   Changing    the   Color   of   Milk. — The   first 

bacterial  fermentation  of  milk  clearly  described  was  that  of  blue 

J  milk,  noticed  and  studied  over  sixty  years  ago.     This 

/%  trouble  has,  therefore,  an  historic  interest  because  of 

x^t  ^^^  connection  with  the  early  development  of  bacteri- 

^'  o\ogy.     It  has  little  practical  interest,  however,  since 

Fig.  38.—     j^  jg  q£  ygj-y  j.g^j.g  occurrence.     It  is  caused  by  a  well- 

1  he  organism  •'  -^ 

producing  known  bacterium  named  B.  cyanogenes  (Fig.  38)  that 
cyanogenes.  ^^^  been  found  in  this  country  as  well  as  in  Europe. 
When  the  organism  is  inoculated  into  milk  it  produces 
no  visible  effect  until  the  milk  is  two  or  three  days  old.  Then  blue 
patches  appear  in  it  that  extend  as  the  milk  sours,  until  the  whole 
becomes  of  a  sky-blue  color.  As  a  dairy  infection  it  is  very  unusual, 
and  no  well-marked  case  of  such  blue  milk  infecting  a  dairy  has 
apparently  been  reported  from  this  country. 

Other  fermentations  producing  pigments  are  also  reported  by 
bacteriologists.  Red  milk  has  occasionally  been  mentioned.  Milk 
may  sometimes  be  red  when  it  is  drawn  from  the  cow  because  of  the 
presence  of  blood,  due  to  some  trouble  in  the  udder.  But  there  are 
also  types  of  red  milk  developing  slowly,  and  due  to  the  growth  of 
bacteria.  B.  prodigiosus,  B.  erylhrogenes,  and  B.  lacto  rubifaciens 
are  three  species  that  have  been  described  as  having  this  power. 
None  of  them  is  of  practical  importance  in  the  dairy.  Red  spots 
in  cheeses  do  sometimes  result  from  the  growth  of  bacteria,  but  red 
milk  is  the  rarest  of  occurrences.  In  addition  to  these  we  some- 
times hear  of  yellow  milk,  orange  milk,  green  milk,  amber-colored 


TYPES  OF  BACTERIA  FOUND  IN  MILK.  157 

milk,  and  black  milk.  I3y  carefully  selecting  the  varieties  of  bacteria, 
and  inoculating  them  into  tubes  of  sterile  milk  there  may  be  produced 
samples  of  milk,  each  showing  different  colors,  all  the  colors  of  the 
rainbow  being  thus  obtained.  All  of  these  phenomena  do  certainly 
occur  in  the  bacteriological  laboratory,  and  all  are  produced  by  the 
growth  of  different  species  of  microorganisms.  But  they  are  usually 
procured  by  inoculating  sterile  milk  with  particular  kinds  of  bacteria 
and  allowing  them  to  act  on  the  milk  for  many  days.  They  are  not 
ordinarily  dairy  phenomena,  and  will  hardly  ever  be  likely  to  ap- 
pear as  dairy  infections.  They  are  of  scientific  rather  than  of 
practical  interest. 

Miscellaneous  Faults. — There  is  a  considerable  list  of  troubles 
appearing  occasionally  in  milk  that  are  due  to  the  growth  of  unusual 
bacteria.  Some  of  these  are  the  following:  Premature  curdling^  the 
milk  curdling  too  quickly  and  without  souring;  failure  to  curdle  at 
all,  even  after  several  days;  had  tastes  such  as  turnip  taste,  rancid 
taste,  putrid  taste;  difficulty  in  churning;  had  tasting  sour  milk;  yeasty 
smell;  soapy  consistency.  These  faults  are  all  unusual  and  in  all  cases 
the  growth  of  unusual  bacteria  is  the  cause.  The  remedy  is  al- 
ways the  same,  more  care  in  cleanliness  and  more  thorough  steriliza- 
tion of  the  milk-vessels.  Any  sample  of  milk  in  which  lactic  acid 
bacteria  fail  to  develop  normally  will  be  sure  to  show  some  trouble 
due  to  the  growth  of  bacteria  that  happen  to  be  present  and  whose 
rapid  growth  is  not  prevented  by  the  acid-foiming  bacteria.  Some- 
times such  troubles  may  be  remedied  by  the  addition  to  the  milk  of 
a  culture  of  ordinary  lactic  acid  bacteria. 

Alcoholic  Fermentation  of  Milk. — Most  sugar  solutions  will 
readily  undergo  an  alcoholic  fermentation,  but  milk  sugar  does  not 
easily  make  this  change.  It  may  be  converted  into  lactic  acid, 
but  not  readily  into  carbon  dioxid  and  alcohol.  Hence  an  alcoholic 
fermentation  of  milk  is  not  a  normal  phenomenon,  although  it  may 
be  pioduced  by  the  addition  of  a  little  cane-sugar  to  the  milk.  The 
possibility  of  making  milk  undergo  an  alcoholic  fermentation  by  the 
addition  of  yeasts  is  made  use  of  in  the  manufacture  of  kummys. 
This  beverage  was  originally  prepared  by  the  Arabs  from  mare's 
milk,  which  will  normally  undergo  an  alcoholic  fermentation;  but 


158  BACTERIA    IN    MILK. 

now  an  imitation  product  is  widely  made  and  used,  prepared  from 
cow's  milk.  A  small  quantity  of  sugar  is  added  to  milk  and  some 
common  baker's  yeast.  An  alcoholic  fermentation  soon  begins, 
and  the  fermented  product  is  kummys.  Various  modifications  of 
this  general  process  are  adopted  by  different  makers,  for  kummys 
has  become  a  commercial  article. 

In  addition  to  these  there  are  several  other  types  of  beverages 
made  from  milk  in  common  use  among  different  nations,  in  the 
production  of  which  alcohol  is  formed.  One,  known  as  kefir, 
has  long  been  used  in  the  Caucasus  mountains.     The  fermentation 


Fig.  39. — A  large-sized  kefir  grain  and  the  three  species  of  bacteria  of 
which  it  is  composed  (Freudenreich). 

is  brought  about  by  adding  to  the  milk  what  are  known  as  kefir 
grains  (Fig.  39).  These  are  hard  nodules  of  various  sizes  which 
have  the  power  of  starting  an  alcoholic  fermentation  in  ordinary 
cow's  milk.  The  origin  of  these  kefir  grains  is  unknown.  To- 
day they  are  handed  from  person  to  person,  taken  from  the  fer- 
mented milk  and  dried  to  be  used  again.  During  the  fermentation 
in  the  milk  they  increase  in  size  and  new  grains  may  be  obtained 
from  fragments  of  the  old  ones.  In  Egypt  the  people  use  a  fermented 
milk  called  leben.  Another,  called  mazoon,  is  common  in  Armenia. 
The  Turks  have  one  they  call  yoghourt,  and  in  Sardinia  still  another 
is  found  with  the  name  of  goiddu.  In  all  these  cases  the  beverage 
is  prepared  by  the  use  of  ferments  that  the  people  keep  on  hand, 
whose  original  sources  are  unknown.     These  ferments,  so  far  as 


GROWTH    OF    BACTERIA    IN    MILK.  1 59 

they  have  been  stud  fed,  prove  to  be  based  upon  the  combined 
action  of  yeasts  and  bacteria.  Very  h'kely  the  bacteria  change 
the  milk-sugar  into  a  fermentable  form  and  at  the  same  time  sour  the 
product.  The  yeast  is  probably  responsible  in  all  cases  for  the 
alcoholic  fermentation  proper,  although  in  some  the  milk  souring 
by  the  bacteria  is  the  primary  feature,  while  the  action  of  the  yeasts 
is  secondary  and  is  not  regarded  by  some  as  at  all  essential  to  the 
product.  In  several  of  these  products  the  type  of  lactic  acid 
organism  mentioned  under  the  name  of  B.  bulgaricus  is  present. 
The  beverages  are  generally  regarded  as  more  digestible  than 
ordinary  milk., 

GROWTH  OF  BACTERIA  IN  MILK. 

The  number  of  bacteria  that  may  be  in  any  sample  of  milk  is, 
in  the  first  place,  dependent  upon  the  number  and  variety  that  get 
into  the  milk  during  and  aftei  the  milking.  But  the  original 
contamination  is  only  a  small  factor  in  determining  their  number 
at  any  subsequent  time.  Milk  furnishes  excellent  food  focJoacteria, 
and  when  drawn  from  the  cow  it  is  warm.  Hence  a  rapid  multi- 
plication of  bacteria  begins;  but  although  the  milk  furnishes  such 
an  excellent  medium  for  them,  they  do  not  begin  to  multiply 
at  once.  For  a  few  hours  their  number  remains  the  same  or  even 
decreases.  There  seems  to  be  something  in  fresh  milk  that  injures 
them.  Whatever  this  may  be,  its  influence  ceases  after  a  few  hours. 
This  power  of  checking  bacteria  growth  is  sometimes  called  the 
germicidal  power  of  milk,  and  it  lasts  from  three  to  twenty-four 
hours,  'according  to  temperature,  being  less  at  higher  temperatures. 
After  it.  has  passed,  the  bacteria  begin  to  increase  rapidly  and 
the  number  present  at  any  later  period  is  more  dependent  upon  the 
extent  of  their  multiplication  than  upon  the  original  contamination. 
The  rate  of  multiplication  of  all  bacteria  depends  upon  temperature. 
The  majority  of  milk  bacteria  grow  best  at  temperatures  between 
60°  and  100°  F.,  and,  generally  speaking,  they  grow  more  rapidly 
at  the  higher  temperatures.  The  effect  of  temperature  is  showni 
by  the  following  example: 


l6o  BACTERIA    IN    MILK. 

Fresh  milk  contained      6,525  bacteria  per  c.c. 

After  25  hours  at  50°  the  same  milk  contained,         6,425  bacteria       " 
After  25  hours  at  70°  the  same  milk  contained,  6,275,000  bacteria       " 

In  this  example  it  is  seen  that  for  twenty-five  hours  the  bacteria 
in  milk  kept  at  50°  did  not  multiply  at  all,  while  in  that  kept  at  70° 
they  multiplied  one  thousand  fold.  It  is  not  common  to  find  such 
a  striking  difference,  but  in  all  cases  there  is  a  very  marked  contrast. 

PROTECTIVE  ACTION  OF  LACTIC  ACID 
BACTERIA. 

If  ordinary  proteids,  like  eggs  or  meat,  are  left  undisturbed  to 
the  action  of  bacteria,  they  will  putrefy.  Milk  also  contains  a 
ptoteid,  casein,  which  is  just  as  liable  to  putrefaction  as  other 
proteids.  But  under  ordinary  conditions  it  does  not  undergo  this 
unpleasant  change.  Milk  sours,  but  rarely  putrefies.  The  reason 
for  this  is  found  in  the  power  of  the  lactic  acid  bacteria  to  restrain 
the  growth  of  other  species.  Almost  from  the  start,  the  lactic  acid 
bacteria  in  milk  grow  more  rapidly  than  the  other  types,  and  as 
they  become  more  abundant,  they  prevent  the  other  kinds  from 
growing;  they  thus  efcctually  restrain  the  growth  of  the  putrefactive 
bacteria,  so  that  milk  that  has  begun  to  sour  will  not  putrefy.  This 
is  really  a  very  useful  function,  for,  whereas  soured,  milk  is  wholesome, 
putrefied  milk  is  not  wholesome,  and  the  lactic  acid  bacteria  thus 
protect  the  milk  from  a  decomposition  which  would  be  far  worse 
than  souring.  It  has  also  come  to  be  a  recognized  fact  that  many 
of  the  troublesome  faults  in  milk  may  be  remedied  by  using  a  culture 
of  lactic  acid  bacteria.  In  cases  of  bitter  milk,  of  premature  curd- 
ling, and  of  other  miscellaneous  troubles,  due  to  undesired  bacteria, 
a  remedy  is  found  by  putting  into  the  milk  a  culture  of  a  vigorous 
lactic  acid  bacterium  that  will  grow  rapidly  and  prevent  the 
undesirable  bacteria  from  developing  sufficiently  to  cause  trouble. 
The  use  of  this  principle  in  butter-  and  cheese-making  has  become 
very  widely  extended. 

It  is  this  restraining  action  of  the  lactic  acid  bacteria  that  explains 
the  generally  recognized  fact  that  sour  milk,  or  butter-milk,  is  not 
only  a  wholesome,  but  a  very  useful  beverage.     It  seems  a  little 


DISEASE    GERMS    IN    MILK.  l6l 

strange  that  these  products,  containing  as  they  do,  bacteria  reckoned 
by  the  hundreds  of  millions  per  c.c,  should  be  recommended  as 
beverages  for  infants  and  invalids.  A  glassful  of  well  soured 
milk  will  certainly  contain  100,000,000,000  bacteria,  and  yet  it 
is  a  wholesome  as  well  as  a  refreshing  beverage.  The  explanation 
however,  is  simple  enough.  These  myriads  of  bacteria  are  practically 
all  of  the  type  of  lactic  acid  bacteria,  which  are  not  only  harmless 
in  themselves,  but  which  prevent  the  growth  of  various  kinds 
of  putrefactive  germs  that  might  produce  trouble  in  the  intestine. 
The  presence  of  a  goodly  number  of  lactic  acid  bacteria,  therefore, 
may  prevent  the  growth  of  certain  types  of  intestinal  putrefaction 
that  would  otherwise  cause  trouble.  The  farmer's  belief  that 
butter-milk  and  sour  milk  are  healthful  drinks,  which  seemed  hardly 
credible  for  a  while  when  the  immense  numbers  of  bacteria  contained 
in  them  were  first  recognized,  appears,  after  all,  to  be  well  founded 
on  scientific  fact.  The  use  of  such  milk  is  becoming  recommended 
very  widely,  and  already  there  are  on  the  market  commercial  prepa- 
rations of  lactic  bacteria  to  be  used  in  preparing  such  milks.  These 
preparations,  as  a  rule,  contain  the  particular  form  of  lactic  bacteria 
mentioned  on  page  148  which  has  the  characteristic  of  being  more 
vigorous,  and  making  milk  more  acid  than  the  ordinary  lactic  acid 
bacteria,  and,  therefore,  having  in  even  greater  degree  this  power  of 
preventing  the  growth  of  other  more  mischievous  organisms.  The 
healthful  properties  ascribed  to  the  alcoholic  beverages  mentioned 
on  page  158  are  probably  due  to  the  presence  of  the  beneficent 
lactic  acid  bacteria. 

DISEASE  GERMS  IN  MILK. 

It  has  long  been  recognized  that  milk  may  be  a  distributer  of 
disease.  This  general  statement  is  disquieting,  but  the  knowledge 
is  of  little  use  unless  it  can  be  made  more  definite.  The  subject 
can  be  made  more  intelligible  if  we  notice  what  kind  of  diseases 
are  thus  distributed  and  how  the  dangers  arise.  There  are  four  defi- 
nite diseases  known  to  be  distributed  in  this  way,  and,  in  addition, 
a  less  definite  type  of  intestinal  trouble. 
14 


1 62  BACTERIA    IN    MILK. 

Tuberculosis. — This  subject  will  be  considered  in  a  separate 
chapter. 

Typhoid  Fever. — Typhoid  fever  is  produced  by  a  well-known 
bacterium  primarily  inhabiting  the  human  intestine  (Fig.  40). 
Inasmuch  as  the  cow  is  not  subject  to  typhoid  fever,  milk,  when 
freshly  drawn,  will  never  contain  typhoid  bacilli.  This  disease, 
therefore,  bears  quite  a  different  relation  to  dairy  matters  from 
tuberculosis.  Milk,  if  infected  with  tuberculosis  bacilli,  contains 
them  when  freshly  drawn,  and  secondary  infection  is 
a  matter  of  no  significance.  But  fresh  milk  never 
contains  typhoid  bacilli,  and  if  they  are  present  in  the 
milk,  they  come  wholly  from  secondary  contamination. 
Fig.  40.—  The  chief  sources  of  these  secondary  contaminations 
bacillul^  °^  ^^^-  ^'  Direct  contact  with  persons  who  have  or  are 
recovering  from  the  disease.  It  is  well  known  that 
patients  may,  after  recovery  from  this  disease,  carry  around  the  living 
bacilli  for  a  long  time;  ^'bacillus  carriers"  they  are  called.  In  other 
cases  the  patient  may  be  so  slightly  sick  with  the  disease  as  to  keep 
about  his  work,  having  what  is  called  "walking  typhoid."  If  people 
from  either  of  these  classes  are  employed  in  the  dairy,  they  will  be 
pretty  sure  to  infect  with  typhoid  fever  germs  whatever  dairy  utensils 
they  handle.  2.  Patients  who  are  sick  enough  to  be  confined  in 
bed  eliminate  large  numbers  of  bacilli  in  their  excretion,  and  this, 
together  with  clothing  soiled  by  it,  may  be  carelessly  handled  by 
some  one  who  is  employed  in  the  dairy.  The  chance  of  milk  infec- 
tion from  such  persons  is,  then,  very  great,  and  no  one  who  has 
anything  to  do  with  the  care  of  a  typhoid  fever  patient  should  be 
allowed  to  have  any  contact  with  the  dairy.  3.  Infected  water  is 
a  common  source  of  contamination.  This  does  not  mean  that 
the  milk  is  necessarily  watered;  but  milk  may  become  infected 
by  simply  allowing  the  cans  to  stand  in  impure  water  while  they  are 
cooling  or  by  rinsing  the  cans  in  such  water  after  they  have  been 
washed.  There  are  also  other  secondary  sources.  That  the  danger 
from  these  sources  is  real  and  not  imaginary,  may  be  judged  from 
the  fact  that  already  at  least  three  hundred  typhoid  epidemics  have 
been  traced  to  milk. 


DISEASE    GERMS    IN    MILK. 


163 


Scarlet  Fever  and  Diphtheria. — There  is  positive  evidence  that 
these  two  diseases  may  be  distributed  by  milk  and  that  some 
epidemics  are  attributable  to  the  milk-supply.  The  cause  of  scarlet 
fever  is  yet  uncertain,  and  it  is  not  known  whether  cows  can  contract 
the  disease  and  then  produce  milk  already  contaminated,  or  whether, 
as  in  typhoid  fever,  the  contamination  of  the  milk  is  wholly  secondary. 
A  few  epidemics  of  scarlet  fever  have  been  traced  to  the  cow  with  more 
or  less  certainty,  and  it  is  beyond  doubt  also  that  the  milk  may 
become  infected  with  the  cause  of  this  disease  by  secondary 
contamination.  The  farmer  should,  therefore,  take 
precautions  to  prevent  any  person  from  working  in* 
the  dairy  who  is  recovering  from  scarlet  fever. 

Diphtheria  is  produced  by  a  well-known  bacillus 
(Fig.  41).    Here  again  there  seems  some  doubt  whether     _  _, 

°  °  Fig.  41.— The 

cows  have  the  disease.  It  is  certain,  however,  that  the  diphtheria  bac- 
milk  may  become  secondarily  infected  through  con-  t^"""^- 
valesccnt  diphtheria  patients  working  in  the  dairy  and  handling  the 
milk.  Some  instances  of  diphtheria  have  been  traced  to  such  a  cause. 
Diarrheal  Diseases. — Besides  the  diseases  mentioned,  milk 
is  responsible  for  a  portion  of  those  obscure  diseases  characterized 
by  diarrheal  troubles,  which  are  especially  prevalent  in  warm 
weather.  Among  these  are  cholera  infantum,  which  is  responsible 
for  the  death  of  so  many  children,  and  summer  complaint,  which  is 
less  serious.  These  troubles  are  not  yet  so  well  understood  as  the 
others  we  have  mentioned.  They  do  not  appear  to  be  caused  by 
any  single  specific  bacterium,  but  are  probably  due  to  the  excessive 
multiplication  of  a  number  of  certain  kinds  of  bacteria  in  the  milk. 
That  they  are  due  to  milk  bacteria  is  proved  by  the  facts  that  (i) 
they  occur  most  frequently  at  the  seasons  of  the  year  when  milk 
bacteria  are  most  numerous;  (2)  they  are  more  prevalent  among  in- 
fants fed  upon  cow's  milk  than  among  breast-fed  children.  What 
kinds  of  bacteria  are  at  fault  in  the  production  of  these  diseases 
we  do  not  know.  Quite  a  number  of  bacteria  are  found  in  milk 
which  produce  poisonous  secretions  and  which  may  be  agents  in 
the  production  of  these  obscure  diseases.  For  the  purpose  of 
our  discussion  it  is  sufficient  to  state  that  they  are  probably  due 


164  BACTERIA    IN    MILK. 

to  putrefactive  bacteria,  which  are  the  bacteria  of  filth.  Anything 
which  increases  the  amount  of  filth  in  the  milk  will  have  a  tendency 
to  increase  the  amount  of  such  troubles,  and  any  advance  in  clean- 
liness will  have  an  influence  in  the  opposite  direction. 


\ 


CHAPTER  XII. 

CONTROL  OF  THE  MILK-SUPPLY. 

A  better  regulation  of  the  milk-supply  is  emphatically  needed,  and 
this  need  has  become  more  and  more  evident  as  the  facts  enumerated 
in  the  last  chapter  have  been  gradually  disclosed.  It  would  enable 
the  dairyman  to  avoid  the  many  troubles  due  to  undesirable  organ- 
isms and  would  be  to  the  public  at  large  a  means  of  protection 
from  the  illnesses  due  to  milk.  In  consequence  of  this  need,  a 
series  of  regulations  and  suggestions  have  arisen  looking  toward 
the  improvement  in  the  quality  of  milk  We  may  best  consider 
these  under  three  heads:  i.  Dairy  pro'  lems.  2.  Transportation 
problems.     3.  Public  control. 

L  DAIRY  PROBLEMS. 

Manifestly  the  first  place  demanding  attention  in  the  attempt 
to  reduce  the  possible  evils  resulting  from  undue  bacterial  contamina- 
tion is  the  dairy.  The  primary  lesson  to  be  learned  here  is  the 
need  of  cleanliness.  But  there  are  several  subordinate  divisions 
of  this  general  subject. 

The  Cow. — The  health  of  the  cow  is  a  matter  of  such  great 
importance  that  it  hardly  needs  to  be  said  that  no  sickly  cow  should 
be  allowed  to  contribute  to  the  milk-supply.  All  tuberculous  cows, 
in  particular,  should  be  excluded,  or  their  milk  used  only  after 
pasteurization.  Every  dairyman  should  be  .on  the  watch  for  udder 
troubles,  and  if  any  signs  of  hardness,  of  inflammation,  or  of  running 
sores  appear  on  the  udders,  or  if  the  animal  gives  bloody  milk, 
she  should  at  once  be  excluded  from  the  milk-produ'cing  herd  until 
completely  recovered.  The  cow  should  also  be  kept  clean.  For- 
tunately, there  has  been  a  decided  change  in  this  respect,  and  at 
the  present  time  cattle  in  dairies  are  not  infrequently  groomed  and 

165 


l66  CONTROL    OF    THE    MILK-SUPPLY. 

brushed,  and  are  sometimes  kept  in  as  cleanly  a  condition  as 
ordinary  horses.  The  habits  of  the  cow,  especially  when  closely 
confined  in  the  stall,  inevitably  result  in  a  large  amount  of  manure 
adhering  to  the  animal's  flanks,  tail,  and  udders,  and  unless  this 
is  removed  by  curry  comb  and  brush,  and  by  washing  if  necessary, 
the  character  of  the  milk  is  sure  to  suffer. 

The  Stables. — It  is  much  better  to  have  stables  on  high  giound, 
where  there  is  ready  drainage,  than  on  low  ground.  Both  air  and 
light  are  necessary  in  stables,  for  the  best  results.  Each  cow 
should  have  three  to  four  square  feet  of  window  surface,  and  400 
to  450  cubic  feet  of  air  space.  While  the  animals  are  in  the  yard, 
as  they  should  be  daily,  the  stables  should  be  thoroughly  aired. 

The  cleanliness  of  the  stable  is  a  matter  of  utmost  importance. 
The  habits  of  the  cow  and  the  nature  of  the  manure  are  such  as 
render  a  high  state  of  cleanliness  very  difficult.  But  the  dairyman 
should  understand  that  all  accumulation  of  manure  or  other  filth  is  a 
direct  detriment  to  the  quality  of  the  milk.  The  removal  of  the 
manure  from  the  stalls  should  be  as  frequent  as  possible,  never  less 
than  twice  a  day.  The  manure,  when  removed,  should  be  taken  as 
far  as  possible  from  the  barn,  and  should  never  be  heaped  outside, 
close  by  the  barn  nor  be  allowed  to  accumulate  in  the  cellar.  By 
far  the  best  method  is  to  distribute  it  daily  upon  the  fields,  where  it 
may  serve  as  a  fertilizer.  Attention  should  be  given  to  the  dust, 
cobwebs  and  hay  that  may  be  clinging  to  the  ceiling  of  the  barn,  for 
all  such  are  traps  for  accumulating  dirt,  as  well  as  sources  of  bacteria, 
thus  aiding  in  the  contaminating  of  the  milk.  Plastered  or  sheathed 
walls  and  ceilings  are  very  much  to  be  preferred  to  a  rough  finish. 
The  bedding  of  the  cattle  is  a  matter  of  some  importance  also;  and 
clean  shavings  appear,  on  the  whole,  the  best  for  this  purpose.  A 
coat  of  whitewash  should  be  applied  with  a  spray  pump  at  least  once 
a  year. 

Personnel. — Special  attention  should  be  given  the  persons  em- 
ployed on  a  dairy  farm.  The  milking  clothes  should  be  made  of 
washable  materials.  Some  dairies  insist  that  this  clothing  must  be 
sterilized  each  day.  A  thorough  washing  and  drying  of  the  hands 
should  precede  the  milking.     All  these  me.asures  are  necessary, 


DAIRY    PROBLEMS.  167 

since  the  persons  employed  in  the  dairy  are  more  likely  to  be  a 
source  of  danger  than  anything  else.  No  one  should  he  allowed  to 
handle  any  milk,  to  wash  the  milk  cans,  or  to  have  anything  whatsoever 
to  do  with  the  milking  utensils  if  he  is  suffering  from  or  recovering 
from  any  contagious  disease.  Nor,  indeed,  should  any  farm  furnish 
milk  to  the  public  if  there  is  a  case  of  typhoid,  scarlet  fever,  or 
diphtheria  among  its  employees,  unless  a  health  inspector  pro- 
nounces the  sanitary  conditions  satisfactory. 

The  Milking-room. — It  is  quite  customary  to  milk  cows  in  the 
ordinary  cow  stalls.  Some  of  the  better  dairies  have  adopted  the 
plan  of  having  a  separate  milking-room,  and  find  beneficial  results  in 
the  character  of  the  milk.  It  is  certainly  preferable  to  using  the 
ordinary  cow  barn  as  a  milking-room. 

The  Milk-vessels. — Perhaps  the  most  important  factor  for 
reducing  bacterial  contamination  is  the  proper  cleaning  of  all  milk- 
vessels.  This  refers  to  milk  pails,  strainers,  coolers,  separators, 
milk  cans,  glass  bottles,  etc.,  used  in  the  dairy.  The  cleaning 
of  such  utensils  is  no  easy  task,  and  after  the  most  thorough 
washing  and  scrubbing  many  bacteria  will  still  be  left  in  the 
cracks  and  clinging  to  the  milk-vessels,  ready  to  feed  and  multiply 
in  the  next  lot  of  milk.  All  milk-vessels  should  be  of  metal,  and 
if  the  coating  of  tin  is  worn  off  they  should  be  discarded,  for  they 
cannot  be  kept  clean.  They  should  not  be  allowed  to  dry  before 
washing,  for  dried  milk  is  difficult  to  remove.  They  should  first  be 
soaked  in  warm  water  to  loosen  the  milk;  then  washed  thoroughly 
in  hot  water,  containing,  preferably,  soap  or  sal-soda,  and  thor- 
oughly scrubbed;  after  this  they  should  receive  a  second  rinsing  in 
hot  water.  Such  a  cleaning  is  not,  however,  sufficient  to  sterilize 
them.  Hence,  no  creamery  should  depend  upon  the  farmer  to  wash 
milk  cans.  Where  a  supply  of  steam  is  to  be  had  a  sterilization 
should  follow  the  washing.  Washing  with  hot  water  is  better  than 
with  cold,  washing  with  sal-soda  is  better  than  simple  washing,  but 
sterilizing  is  best  of  all.  Each  dairyman  should  adopt  as  thorough  a 
cleaning  as  parcticable. 

The  Milking. — Moistening  the  udder  with  a  damp  cloth  or 
sponge  just  before  milking  prevents  the  fall  of  much  of  the  dirt  into 


1 68 


CONTROL    OF    THE    MILK-SUPPLY. 


the  milking  pail.  The  precaution  is  a  simple  one,  costs  nothing  and 
really  has  a  surprising  result  in  decreasing  the  number  of  bacteria 
that  get  into  the  pail. 

Covered  Milk-pail. — The  old  fashioned  pail  had  a  flaring  top, 
the  purpose  of  which  was  to  make  the  milking  as  easy  as  possible; 
but,  incidentally,  it  resulted  in  exposing  the  milk  to  much  contami- 
nation by  dirt  and  bacteria.     Various  devices  for  protecting  the 
milk  from  such  exposure  by  the  use  of  covered  milk-pails  are  now 
used.     There  is  quite  a  variety  among  them,  but  they  all  have  the 
general  plan  of  decreasing  the  size  of  the 
^"j^w^oB^^^^        opening  of  the  milk- vessels,  so  as  to  expose 
Wt^  j^m  ^/Bf         ^^^^  surface  for  the  entrance  of  dirt,  and 
V    ■^- — ^'^^^^B  they  also  have  in  the  opening  some  kind  of 

'  '^^  a    cloth    strainer    for   catching   the    larger 

particles  of  dirt,  thus  keeping  them  from 
the  milk  (Fig.  42).  This  is  one  of  the 
easiest,  cheapest  and  most  efficient  means 
for  improving  the  character  of  the  milk. 

Milking  Machines. — A  still  more  recent 
means  of  reducing  contamination  is  by 
milking  machines.  These  consist  of  rubber 
tubes  ending  in  special  cups  for  attachment 
to  the  teats  of  the  cow,  and  connected  at 
the  other  end  with  large  cans  that  can  be  sterilized.  The  cans 
are  connected  with  a  system  of  vacuum  tubes,  and  at  the 
point  where  the  rubber  tubes  are  attached  to  the  can  there  is  a 
mechanical  device  by  which  the  vacuum  is  made  to  draw  the  milk 
through  the  tubes  intermittently,  thus  imitating  natural  milking. 
It  would  seem  that  such  a  plan,  which  carries  milk  directly  from 
the  teat  to  the  sterilized  can,  would  be  almost  ideal,  and  would 
practically  remove  all  dirt  contamination.  Where  these  machines 
have  been  intelligently  used  they  have  been  found  efficient  in  pro- 
ducing a  very  clean  quality  of  milk.  But  the  long  rubber  tubes  are 
by  no  means  easy  to  keep  clean,  and  when  they  are  used  by  careless 
employees,  the  bacteria  become  very  abundant  inside  the  tubes  and 
the  other  parts  of  the  somewhat  complicated  machine.     In  other 


Fig.  42. — A  milk  pail 
with  a  special  cover  designed 
to  keep  out  the  dust  which 
falls  into  the  pail  durinj 
milking. 


DAIRY    PROBLEMS.  1 69 

words,  it  requires  very  great  care  to  clean  and  sterilize  these  milking 
machines  in  order  to  produce  even  as  good  results  as  are  obtained 
by  hand  milking.  But  if  care  be  taken  to  sterilize  thoroughly  all  of 
the  apparatus,  better  and  more  reliable  milk  can  be  obtained  by  the 
use  of  the  milking  machine. 

Rejecting  Fore  Milk. — ^For  reasons  already  indicated,  the  first  milk 
drawn  at  each  milking  will  contain  more  bacteria  than  the  rest. 
The  practice  of  rejecting  the  fore  milk,  either  allowing  it  to  waste 
upon  the  floor,  or  collecting  it  in  a  separate  dish,  is,  no  doubt,  an 
advantage,  but  the  extent  of  the  advantage  has  been  overdrawn. 
The  extra  number  of  bacteria  obtained  in  a  pail  of  milk  from  the 
entrance  of  the  fore  milk,  is  very  small  compared  with  the  larger 
number  that  enter  the  milk  from  other  sources. 

Value  of  Trained  Dairymen. — Apparatus  without  a  proper  man 
to  use  it  is  valueless.  It  makes  no  difference  how  many  rules  may 
be  drawn  concerning  the  dairy,  how  complicated  the  apparatus 
becomes,  or  how  careful  may  be  the  directions  given  to  the  em- 
ployees, it  is  quite  impossible  to  expect  satisfactory  results  without 
properly  educated  and  trained  assistants.  An  untrained  man  will 
succeed  in  getting  only  bad  results,  even  with  the  best  of  apparatus. 
The  employees  in  our  dairies  at  present  are,  in  many  cases,  without 
any  proper  training.  They  do  not  know  the  character  of  the  prod- 
uct they  are  producing;  they  do  not  know  the  dangers  to  which  it 
is  subject;  they  do  not  understand  the  universal  presence  of  bacteria; 
they  do  not  understand,  in  general,  the  pi:oblems  that  are  concerned 
with  their  business.  They  are  quite  likely  to  believe  the  whole 
subject  of  bacteria  in  milk  to  be  foolishness  and  not  worth  their 
attention.  Under  these  circumstances,  no  matter  how  many 
directions  are  given  or  how  much  instruction  there  may  be,  satis- 
factory results  will  never  be  obtained. 

Cooling. — The  importance  of  cooling  the  milk,  and  cooling  it 
immediately,  cannot  be  overstated.  When  milk  is  drawn  from  the 
animal,  it  is  at  a  temperature  to  stimulate  the  growth  of  bacteria  to 
their  utmost.  It  is  true  that,  for  a  while,  because  of  the  germicidal 
property  of  milk  (see  page  159),  the  bacteria  do  not  grow;  but  this 
condition  lasts  only  a  short  time,  after  which,  if  the  milk  is  warm, 
IS 


170  CONTROL    OF    THE    MILK-SUPPLY. 

they  begin  to  develop  with  great  rapidity.  But  if  the  milk  is  at  once 
reduced  to  a  low  temperature,  the  bacteria  that  have  found  their 
way  into  the  milk  will  not  grow  very  rapidly.  These  facts  are  so 
simple  as  hardly  to  require  statement;  but,  unfortunately,  many  a 
dairyman,  although  he  may  theoretically  understand  them,  fails  to 
appreciate  their  importance.  The  essential  point  to  be  emphasized 
is  the  necessity  of  immediately  cooling  the  milk  to  a  temperature  as 
low  as  40°  F.  if  possible.  It  is  just  as  necessary  to  cool  clean  milk 
as  it  is  to  cool  dirty  milk.  Unless  it  is  done,  the  cleanest  milk  will 
soon  contain  as  many  bacteria  as  the  dirtiest  milk. 

Straining  and  Filtering. — The  long-continued  practice  of 
straining  the  milk  through  a  metal  strainer  or  through  cloth  has  in 
its  favor  the  fact  that  it  will  remove  the  larger  particles  of  dirt;  but 
it  does  not  remove  the  bacteria,  for  they  will  pass  through  any 
strainer.  Sand  filters  have  also  been  used  by  some  dairy  com- 
panies, and  these  are  more  efficient  than  simple  straining.  But 
these  filters  are  not  of  very  much  value  and  they  are  not  widely 
used.  Centrifugal  force  is  sometimes  used  for  cleaning  the  milk, 
and  is  fully  as  efficient  as  sand  filtering.  All  of  these  means,  while 
effective  in  removing  the  large  particles  of  dirt,  are. practically  of 
no  value  in  removing  the  bacteria,  which  show  as  high  numbers 
after  such  treatment  as  before. 


II.  TRANSPORTATION  PROBLEMS. 

Under  this  head  will  be  included  not  only  methods  of  treating 
milk  during  transportation,  but  also  of  preparing  it  for  preservation 
during  the  transportation  or  until  it  is  consumed.  Milk,  as  a  rule, 
receives  no  preparation  for  transportation,  except  that  of  cooling  and 
placing  in  clean  cans.  Then,  if  rapidly  shipped  and  kept  cool,  it 
should  remain  good  until  some  time  after  it  has  reached  the  con- 
sumer. But  the  rapidity  of  bacteria  growth,  especially  in  hot 
weather,  makes  it  difficult  to  transport  milk,  in  good  condition,  for 
very  long  distances.  Consequently,  careful  search  has  been  made 
for  some  method* of  treating  milk  so  as  to  preserve  it. 


TRANSPORTATION    PROBLEMS.  171 

The  Use  of  Preservatives. — It  is  easy  to  add  to  the  milk 
various  chemicals  which  will  prevent  the  growth  of  bacteria,  and 
consequently  preserve  the  milk.  Many  such  substances  have 
been  used.  There  are  quite  a  number  of  preservatives  on  the 
market  which  are  sold  to  the  farmer  to  assist  him  in  preserving  his 
milk.  The  basis  of  most  of  these  is  either  horacic  acid,  salicylic  acid, 
or  formalin.  All  of  these  substances  are  injurious  to  man,  and 
their  use  should  not  be  allowed  in  preserving  an  article  so  freely 
used  as  milk.  Such  methods  are  illegal,  and  are  unhesitatingly  to 
be  condemned. 

The  Use  of  Heat. — A  more  legitimate  method  of  obtaining  the 
same  result  is  by  the  use  of  heat.  All  bacteria  are  destroyed  by  heat 
and  therefore,  by  this  simple  means,  it  is,  possible,  to  kill  the  living 
organisms  in  milk,  and  thus  preserve  the  milk  from  their  subsequent 
action.  This  has  given  rise  to  two  chief  methods  of  treating  milk — 
sterilization  and  pasteurization. 

I.  Sterilization. — This  means  the  use  of  heat  sufficient  to  de- 
troy  all  bacteria  at  once.  It  is  perfectly  possible  to  do  this,  but  since 
milk  always  contains  spore-bearing  bacteria,  sterilization  requires  a 
high  temperature  for  the  purpose.  A  temperature  of  boiling  will 
not  destroy  the  spores,  so  it  is  necessary  to  heat  the  milk  to  several 
degrees  above  boiling.  This  involves  the  use  of  special  apparatus, 
in  which  bottles  of  milk  can  be  inclosed  in  special  vessels,  sub- 
jected to  steam  under  pressure,  and  subsequently  hermetically 
sealed  while  still  within  the  closed  vessels.  Such  a  procedure 
inevitably  makes  the  milk  rather  expensive.  But  milk  thus  pre- 
pared is  supposed  to  be  germ-free,  and,  consequently,  should  keep 
indefinitely.  Unfortunately,  even  these  temperatures  do  not  al- 
ways destroy  all  the  spores,  for  some  samples  of  milk  thus  treated 
have  subsequently  undergone  fermentative  changes,  due  to  the 
germination  of  the  spores  that  are  left  alive.  Further,  it  has  ap- 
peared that  these  later  changes,  due  to  the  resisting  spores,  are 
frequently  such  as  do  not  change  the  appearance  of  the  milk  to  the 
eye,  so  that  such  milk,  though  containing  bacteria  in  quantity,  will 
be  drunken  as  pure  milk.  The  fermentation  has,  moreover,  filled 
the  milk  with  bacterial  products  of  more  or  less  injurious  nature, 


172  CONTROL    OF    THE    MILK-SUPPLY. 

and  consequently  the  drinking  of  such  milk  is  far  worse  than  drinking 
fresh  milk  which  is,  most  likely,  supplied  chiefly  with  lactic  bacteria. 
Sterilized  milk,  if  it  does  retain  a  single  spore,  will  be  in  time,  more 
dangerous  than  ordinary  fresh  milk.  For  this  reason,  among 
others,  this  practice  of  treating  milk  to  superheated  steam  for  the 
purpose  of  absolute  sterilization  has  disappeared. 

The  term  sterilization  is  sometimes  applied  to  the  simple  boiling 
of  milk.  This  was  recommended  by  physicians  long  before  its 
real  significance  was  understood  and  has  been  very  widely  used  in  all 
civilized  countries.  Its  ease  of  application  explains  the  reason  for 
its  popularity.  It  is  only  necessary  to  place  the  milk  upon  the 
stove  and  allow  it  to  come  to  a  boil,  and  the  end  is  reached.  In 
some  countries  very  little  milk  is  used  without  such  previous  boiling, 
and  even  the  children  are  taught  in  school  that  it  is  dangerous  to 
drink  milk  without  such  treatment.  The  purpose  aimed  at  in  this 
wide  use  of  boiling,  which  is  commonly,  though  not  properly, 
called  "sterilization,"  is  simply  to  destroy  the  danger  of  distributing 
diseases  by  the  destruction  of  pathogenic  bacteria.  This  purpose 
is  certainly  achieved,  for  the  boiling  temperature  does  destroy  all  the 
pathogenic  bacteria  which  are  likely  to  be  in  milk,  since  none  of 
these  are  spore  producers. 

But  several  practical  objections  have  arisen: 

1.  The  bacteria  spores  are  not  destroyed,  and  such  milk,  if  kept, 
will  surely  undergo  a  fermentation.  But  this  is  of  little  importance 
if  the  boiled  milk  is  to  be  used  at  once. 

2.  The  milk  acquires  the  well-known  taste  of  boiled  milk  which 
is,  to  most  people,  unpleasant.  People  are  willing  to  take  boiled 
milk  upon  an  emergency  as  an  ivalid  diet,  but  few  will  continue  its 
use.  The  taste  is  not  enjoyed,  and,  rather  than  drink  boiled  milk, 
the  majority  of  people  will  give  up  drinking  milk  altogether.  This 
is  certainly  not  desirable,  since  milk  forms  one  of  the  best  and 
cheapest  foods.  Any  treatment  which  greatly  reduces  the  amount 
used  is,  in  itself,  undesirable;  and  the  practice  of  boiling  milk 
certainly  does  reduce  the  amount  used. 

3.  Milk  treated  to  a  temperature  as  high  as  boiling  becomes 
somewhat  less  easy  of  digestion  and  assimilation.     The  heat  pro- 


TRANSPORTATION    PROBLEMS.  1 73 

duces  several  important  changes  which  result  in  its  being  less  easily 
handled  by  the  digestive  organs.  The  difference  is  not  very  great, 
and  a  healthy  individual  is  able  to  digest  such  milk  well  enough; 
but  delicate  children  and  invalids  are  not  so  well  nourished  upon 
boiled  milk  as  upon  raw  milk. 

4.  Boiling  the  milk  is  a  treatment  which  has  not  proved  practical 
to  adopt  on  a  large  scale  at  a  central  source  of  supply.  It  offers, 
therefore,  no  assistance  either  to  the  producer  or  to  the  distributer 
in  enabling  him  to  furnish  milk  which,  since  it  keeps  longer,  gives 
greater  satisfaction. 

The  practice  of  boiling  milk  in  order  to  "sterilize"  it  is  widely 
adopted  in  private  families,  but  the  objections  urged  against  it 
have  led  to  its  being  less  and  less  recomm^ended.  In  its  place  has 
come  an  extended  use  of  the  second  method  of  treating  milk  by 
heat. 

2.  Pasteurization. — This  method,  originally  devised  by  Pasteur 
for  treating  wine,  consists  in  heating  the  milk  to  a  moderate  tempera- 
ture only,  and  then  rapidly  cooling  it.  The  temperatures  chosen 
have  varied.  Sometimes  a  temperature  as  low  as  140°  F.  is  adopted, 
and  continued  for  from  twenty  minutes  to  half  an  hour  or  more; 
sometimes  150°  to  160°  is  used  for  about  ten  minutes,  and  some- 
times as  high  a  temperature  as  180°  is  used,  the  milk  being  just 
brought  to  this  point  and  then  cooled  at  once.  Any  of  these 
methods  is  called  pasteurization. 

Such  temperatures  are  manifestly  not  sufficient  to  sterilize  the 
milk,  since  they  are  even  less  efficient  than  boiling.  But  in  pasteuri- 
zation no  attempt  is  made  to  sterilize  it,  but  simply  (i)  to  destroy  the 
large  majority  of  bacteria  and  (2)  to  destroy  the  disease  germs 
that  are  liable  to  be  in  the  milk,  and  therefore  to  render  it  safe  for 
drinking.  A  low  temperature  is  chosen  in  order  to  avoid  the  chemi- 
cal changes  that  are  pioduced  by  boiling  and  that  show  themselves 
in  the  boiled  taste.  These  changes  begin  to  appear  at  about  156°, 
and  any  temperature  below  this  scarcely  changes  the  milk  at  all, 
while  higher  temperatures  will  bring  them  about.  For  this  reason 
the  lower  temperatures  are  better.  But  will  these  moderate 
temperatures  accomplish  the  desired  ends  ? 


174  CONTROL    OF    THE    MILK-SUPPLY. 

Such  moderate  temperatures  certainly  do  increase  the  keeping 
quality  of  the  milk.  While  a  temperature  of  156°  F.  does  not 
destroy  spores,  it  does  very  largely  destroy  the  active,  non-spore- 
bearing  bacteria.  Now  the  lactic  acid  bacteria,  which  are  the  cause 
of  the  souring  of  milk,  produce  no  spores,  and  consequently  they  are 
largely  killed  by  such  moderate  heat.  Hence  the  total  number  of 
bacteria  in  milk  is  immensely  reduced,  and  the  milk  has  its  keeping 
quality  much  increased.  Milk  thus  treated  will  frequently  remain 
good  two  days  longer  than  similar  milk  not  pasteurized. 

Will  such  temperatures  destroy  disease  bacteria?  Of  the 
diseases  mentioned  above  as  liable  to  distribution  by  means  of  milk, 
there  is  only  one  in  regard  to  which  there  has  been  any  disagreement. 
It  is  admitted  on  all  sides  that  typhoid  and  diphtheria  bacteria 
are  killed  by  the  low  heat  (140°  F.  for  one-half  hour);  the  same 
is  probably  true  of  scarlet  fever.  The  tuberculosis  bacillus,  how- 
ever, will  withstand  higher  heat  without  injury,  and  hence,  in  order 
to  be  sure  of  destroying  these  organisms,  it  has  been  thought  neces- 
sary to  heat  the  milk  to  temperature  of  185°  F.  At  this  temperature 
the  cooked  taste  and  the  chemical  changes  begin  to  appear.  The 
piesent  conclusion,  the  result  of  the  most  recent  and  careful  experi- 
menting, is  happily  a  satisfactory  one.  If  milk  is  heated  in  such 
a  manner  as  to  avoid  the  formation  of  a  scum  on  its  surface,  at  a 
temperature  no  higher  than  140°  F.,  but  continued  for  half 
an  hour,  the  virulence  of  the  tubercle  bacillus  will  be  so  much 
reduced  that  milk  containing  these  bacilli  will  be  rendered  harmless. 
This  temperature  is  considerably  below  that  at  which  the  chem- 
ical changes  in  -the  milk  take  place.  Milk  may  thus  be  deprived 
of  its  danger  of  distributing  disease  germs  without  having  its 
physical  or  chemical  nature  noticeably  changed.  Such  milk, 
when  cooled,  cannot  be  distinguished  from  fresh  milk. 

The  value  of  pasteurization  is  becoming  rapidly  recognized,  and 
this  method  of  treatment  is  being  widely  adopted.  The  advantages 
lie  in  the  following  facts: 

1.  It  produces  milk  which  cannot  be  distinguished  from  fresh 
milk  and  will  be  used  as  freely. 

2.  It  increases  the  keeping  propeity  of  the  milk,  but  not  to  the 


TRANSPORTATION    PROBLEMS.  1 75 

extent  of  leading  the  consumer  to  believe  he  can  keep  it  indefinitely. 
The  consumer  is  thus  forced  to  use  it  up  before  the  spore-bearing 
bacteria  get  an  opportunity  of  multiplying  sufficiently  to  produce 
the  injurious  secretions  which  occasionally  render  sterilized  milk 
dangerous.  The  very  fact  that  the  method  does  not  destroy  all 
bacteria  is  a  safeguard. 

3.  It  removes  the  danger  of  distributing  pathogenic  bacteria. 
This  is  certainly  quite  true  of  the  typical  diseases  mentioned. 
Whether  it  similarly  removes  the  danger  of  diarrheal  diseases, 
not  dependent  upon  any  known  specific  bacteria,  is  not  yet  positively 
known  by  experiment,  inasmuch  as  we  do  not  know  the  actual 
cause  of  the  diseases.  But  the  practical  experience  of  physicians 
tells  us  that  pasteurized  milk  acts  as  efficiently  as  sterilized  milk 
in  reducing  these  diseases. 

4.  This  method  of  treatment  is  perfectly  applicable  upon  a 
large  scale.  Several  forms  of  apparatus  have  been  devised  that 
accomplish  the  end  rapidly  and  upon  large  quantities  of  milk. 
Of  these,  there  are  two  general  types.  In  one  a  large  quantity  of 
milk  is  heated  to  the  desired  temperature  and  maintained  at  this 
temperature  as  long  as  desired,  after  which  it  is  cooled.  These 
are  called  discontinuous  pasteurizers.  In  the  other  type  the  milk 
is  passed  through  the  apparatus  in  a  constant  stream,  being  heated 
and  cooled  while  it  passes  through.  In  these  machines  the  milk 
is  sometimes  only  just  brought  to  the  desired  temperature,  and 
cooled  at  once;  and  in  all  cases  the  extent  of  the  heating  is  dependent 
upon  the  rapidity  of  the  stream  flowing  through.  These  are  called 
continuous  pasteurizers.  Generally  speaking,  this  type  is  apt  to 
be  less  efficient  than  the  discontinuous  pasteurizers,  and  are  more 
subject  to  irregularity.  Either  type  is  efficient  if  properly  managed, 
but  carelessness  and  haste  on  the  part  of  the  employees  may  render 
either  kind  unreliable  and  inefficient. 

In  the  last  few  years  the  plan  of  pasteurizing  the  milk  on  a  large 
scale  has  come  to  be  frequently  adopted.  It  is  done  in  creameries 
in  connection  with  butter-making,  and  in  some  of  our  large  cities 
for  the  treatment  of  the  general  milk-supply.  In  the  pasteuriza- 
tion of  the  public  milk-supply  the  purpose  has  not  been,  primarily,  to 


176  CONTROL    OF    THE    MILK-SUPPLY. 

protect  the  public,  but  to  keep  the  milk  from  souring.  Milk 
distributers  have  found  it  difficult  to  furnish  milk  that  will  keep 
without  preservatives,  but  have  learned  that  the  application  of  heat 
enables  them  to  do  so.  For  this  reason  pasteurization  has  become 
adopted  by  some  large  milk  companies. 

Pasteurization  is  sometimes  applied  to  cream,  to  enhance  its 
keeping  and  enable  it  to  find  a  market.  The  cream  keeps  well, 
but  loses  some  of  its  consistency.  It  appears  thinner  than  before 
treatment,  and  will  not  whip  so  well  as  ordinary  cream.  Its  consist- 
ency may  be  restored  by  adding  a  little  of  a  material  called  viscogen. 
This  is  made  by  adding  a  strong  solution  of  cane-sugar  to  freshly 
slacked  lime,  and  allowing  the  mixture  to  stand  until  the  upper  part 
of  the  mixture  is  clear.  This  clear  liquid  is  poured  off  and  added 
to  the  cream  in  the  proportion  of  one  part  to  one  hundred  or  one 
hundred  and  fifty  parts  of  cream.  This  restores  the  consistency  to 
the  cream,  but,  since  it  is  an  addition  of  a  foreign  substance,  its 
use  is  illegal. 

In  our  larger  cities  a  considerable  part  of  the  milk  on  the  market 
is  pasteurized;  sometimes  it  is  sold  as  pasteurized  milk  and  some- 
times it  is  not  so  labeled. 

Preparations  of  Milk. — The  microorganisms  that  spoil  milk 
will  not  grow  in  it  if  the  water  is  removed,  and  several  methods  have 
been  devised  for  producing  a  form  of  milk  that  will  keep,  all  of  which 
are  based  upon  the  removal  of  the  water.  Condensed  milk  is  the 
oldest  and  has  a  wide  use.  It  consists  of  ordinary  milk  evapo- 
rated to  about  one-third  of  its  original  bulk,  to  which  is  com- 
monly added  a  large  amount  of  sugar.  The  sugar  prevents  the 
growth  of  bacteria,  and  this  condensed  milk,  put  up  in  cans,  keeps 
well.  In  some  forms  of  condensed  milk  the  sugar  is  not  added,  but 
the  product  is  preserved  by  sterilizing  by  heat.  When  subsequently 
diluted  with  water,  condensed  milk  does  not  exactly  replace  the 
fresh  article,  because  of  the  added  sugar  in  the  one  type  and  the 
effect  of  sterilization  in  the  other.  A  product  known  as  concen- 
trated milk  has  recently  been  placed  on  the  market.  In  this  case 
the  milk  is  first  skimmed  and  then  subjected  -to  a  heat  of  140°  F. 
till  enough  water  is  evaporated  to  bring  the  milk  to  about  one-fifth 


TRANSPORTATION    PROBLEMS.  1 77 

of  its  original  bulk.  Then  the  cream,  which  has  also  been  sub- 
jected to  the  same  heat,  is  replaced,  making  a  product  one-fourth  its 
former  weight.  This  milk  has  been  pasteurized  by  the  heat  used  in 
evaporation  and  is  consequently  free  from  disease  bacteria.  When 
the  original  amount  of  water  is  replaced  it  is  indistinguishable  from 
fresh  milk.  Concentrated  milk  will  keep  for  several  days  without 
spoiling,  and  can  be  much  more  easily  handled  than  ordinary  milk. 
In  still  other  preparations  the  water  is  almost  wholly  removed,  pro- 
ducing milk  powders,  several  different  brands  being  on  the  market. 
They  are  prepared  by  various  means,  but  in  all  the  water  is  dried 
away  from  the  milk,  leaving  a  form  that  can  be  converted  into  a 
powder.  Since  they  contain  little  water  they  will  keep  almost  in- 
definitely. They  have  great  use  for  special  purposes,  but  are  not 
a  satisfactory  substitute  for  fresh  milk  since  they  do  not  readily  dis- 
solve again  when  water  is  added  to  them. 

Transportation. — In  the  transportation  of  milk  to  market 
three  factors  are  to  be  borne  in  mind:  i.  Cleanliness.  This  means 
that  only  thoroughly  sterilized  cans  should  be  used  to  hold  the  milk, 
and  that  they  should  be  completely  closed,  so  as  to  avoid  contamina- 
tion from  without.  The  necessity  for  a  complete  sterilization  of  the 
milk  cans  cannot  be  exaggerated.  2.  Temperature.  If  the  milk  is 
to  be  delivered  in  a  good  condition,  it  must  be  kept  cold  during 
transportation.  This  is  accomplished  fairly  well  by  the  ice  car. 
3.  Rapidity.  The  more  quickly  milk  can  be  delivered  to  the  cus- 
tomer, the  better  the  result.  But  milk  kept  cold,  below  45° F.,  may 
be  delivered  from  24  to  36  hours  old  and  be  in  better  condition 
than  milk  fresh  from  the  farm,  only  five  or  six  hours  old,  which  has 
not  been  properly  cooled.  For  this  reason  it  not  infrequently 
happens  that  milk  brought  in  a  milk  cart,  directly  from  the  farm 
only  a  few  miles  distant  from  the  consumer,  is  of  poorer  quality,  so 
far  as  numbers  of  bacteria  are  concerned,  than  milk  that  has  been 
brought  long  distances  in  a  well  iced  milk  car,  though  it  may  be 
36  or  even  48  hours  old.  As  an  actual  fact,  milk  furnished  small 
communities  near  the  source  of  the  supply  is  frequently  of  poorer 
quality  and,  on  the  whole,  less  reliable  than  that  furnished  the 
larger  cities. 


178  CONTROL    OF    THE    MILK-SUPPLY. 

III.  PUBLIC  PROBLEMS. 

Although  this  subject  primarily  concerns  the  regulation  of  the 
milk-supply  after  it  reaches  the  city,  certain  aspects  of  it  are  inti- 
mately associated  with  farm  life.  City  authorities  are  every  year 
extending  their  control  more  and  more  directly  to  the  farm.  The 
public  is  making  certain  demands  regarding  the  milk-supply  that 
must  be  acceded  to  by  the  milk  producer. 

Freedom  from  Disease  Germs. — This  demand  needs  no  argu- 
ment. To  meet  it  the  only  plan  within  sight  at  present  is  the  in- 
sistence that  only  healthy  cows  shall  be  used  in  the  production  of 
milk;  that  no  milk  shall  be  distributed  for  drinking  purposes  from 
cows  having  any  kind  of  udder  disease;  that  no  person  suffering 
from  or  recovering  from  a  contagious  disease,  or  having  direct  con- 
tact with  others  thus  suffering  shall  be  employed  in  the  dairy  or 
handle  the  milk  in  any  way;  that  the  milk  shall  not  be  watered,  and 
that  in  washing  the  milk- vessels  no  water  shall  be  used  that  is  in  the 
slightest  degree  open  to  suspicion  of  sewage  contamination.  In 
addition  to  these  demands  it  must  be  insisted  that  precautions  be 
taken  for  excluding  stable  filth  from  the  milk,  and  that  the  milk  be 
cooled  at  once  to  prevent  undue  growth  of  bacteria. 

Milk  Standards. — A  legal  standard  set  for  the  chemical  com- 
position of  market  milk  is  nearly  everywhere  adopted  and  does  not 
concern  our  immediate  subject.  A  few  cities  have  set  a  standard 
as  to  the  number  of  bacteria  that  will  be  allowed  in  milk  offered  for 
sale.  Boston  has  a  standard  of  500,000  per  c.c.  This  has  as  yet 
been  done  in  only  a  few  places  and  it  is  still  uncertain  whether  such 
standards  can  be  enforced  or  are  of  much  value.  The  milk  pro- 
ducer needs  only  to  remember  that,  to  reach  these  standards,  he 
must  use  care  in  the  dairy  to  insure  cleanliness  along  lines  already 
pointed  out.  Special  grades  of  milk  are  becoming  more  or  less 
common  in  various  localities.  Sanitary  dairies  of  exceptionally 
high  character  have  been  conducted  with  more  or  less  success.  In 
these  every  possible  precaution  is  adopted  to  produce  milk  under 
ideal  conditions.  Only  tested  and  inspected  cows  are  used,  and 
numerous  devices  are  carried  on  to  protect  the  milk  from  all  possible 


PUBLIC    PROBLEMS. 


179 


suspicion  of.  filth  contamination.  The  milk  from  these  dairies  is 
certainly  superior  to  the  ordinary  milk,  but  the  production  is  so 
expensive  that  it  must  be  sold  at  a  high  price,  and  this  has  interfered 
with  the  commercial  success  of  some  of  these  enterprises.  What  is 
known  as  certified  milk  has,  in  recent  years,  come  into  some  promi- 
nence. This  is  milk  produced  in  dairies  that  are  under  the  inspec- 
tion of  a  certifying  board.  This  board,  usually  composed,  in  part  at 
least,  of  physicians,  keeps  a  constant  oversight  of  the  milk  from  cer- 
tain dairies  and  over  the  methods  of  its  production.  If  they  find  that 
the  milk  comes  up  to  the  somewhat  high  standard  that  they  set,  and 
if  they  are  convinced  that  proper  methods  are  used  in  its  production, 
this  board  gives  to  that  dairy  the  right  to  use  its  certificates.  It  is 
not  so  expensive  to  produce  milk  under  these  conditions  as  to  carry 
out  the  many  precautions  adopted  by  the  sanitary  dairies.  Some 
extra  care  is  needed,  but  it  is  within  the  reach  of  almost  any  well 
kept  farm  to  produce  certified  milk.  This  milk  brings  a  higher 
price  than  ordinary  milk,  but  it  is  more  reliable  because  more  care 
has  been  required-  to  produce  it.  Neither  sanitary  milk  nor  certi- 
fied milk  forms  anything  more  than  a  very  small  portion  of  the  milk- 
supply  of  our  cities. 

Dairy  Inspection. — During  recent  years  the  practice  of  inspect- 
ing dairies  has  sprung  up.  This  was  started  first  by  some  of  the 
milk-supply  companies  of  the  large  cities,  because  they  wished  to 
protect  their  supply  for  commercial  purposes.  Some  of  them  began, 
at  least  a  dozen  years  ago,  to  send  inspectors  periodically  among  the 
dairies  which  furnished  them  with  milk.  Within  a  few  years  it 
has  been  realized  that  a  public  dairy  inspection  of  this  sort  would 
be  of  great  value  in  improving  the  general  milk-supply  and  in 
furnishing  the  public  with  better  milk.  Such  a  public  dairy  inspec- 
tion has  been  begun  in  some  sections  around  the  larger  cities. 
The  inspectors  visit  the  farms,  note  all  methods  employed,  condemn 
the  faulty  ones  and  make  suggestions  as  to  improvement.  The 
inspection  is  for  the  advantage  of  the  consumer  and  producer, 
and  the  dairyman  should  welcome  rather  than  resent  such  visits 
and  helpful  suggestions. 

The   inspectors   give   attention   to    the    following    points:     i. 


l8o  CONTROL    OF    THE    MILK-SUPPLY. 

General  cleanliness  in  the  dairy.  2.  The  condition  of  the  cows. 
3.  The  source  of  the  dairy  water.  4.  The  condition  of  the  barn. 
5.  The  method  of  disposing  of  the  manure.  6.  The  method  of 
milking.  7.  The  condition  of  the  milkers.  8.  The  treatment  of 
the  milk  after  milking.  9.  The  method  of  washing  and  sterilizing 
all  dairy  utensils.  10.  The  botthng  of  the  milk.  11.  The  method 
and  care  of  transportation.  The  farmer  should  be  prepared  to 
meet  the  inspector  upon  all  these  points. 

While  it  is  expensive  to  produce  sanitary  milk,  and  while  the 
same  is  true,  though  to  a  less  extent,  of  the  production  of  certified 
milk,  it  is  possible  greatly  to  improve  the  character  of  the  milk 
without  any  material  increase  in  expense.  The  purpose  of  dairy 
inspectors  is  to  show  how  the  milk  may  be  improved;  and  the  dairy- 
man should  remember  that  by  the  use  of  such  simple  precautions 
as  cleaning  the  cow,  moistening  the  udder  before  milking,  using 
a  covered  milking  pail,  and  more  thoroughly  sterilized  milk-vessels, 
the  character  of  the  milk  will  be.greatly  improved.  It  is  gratifying 
to  know  that  there  has  been  a  decided  improvenlent  in  the  quality 
of  milk  furnished  our  cities  in  recent  years. 


CHAPTER  XIII. 
BACTERIA  IN  BUTTER  AND  OLEOMARGARINE. 

BACTERIA  IN  BUTTER-MAKING. 

In  the  making  of  butter,  bacteria  are  the  dairyman's  allies.  The 
butter-maker  always,  even  though  unconsciously,  makes  direct  use 
of  bacteria  when  he  subjects  his  cream  to  a  process  almost  univer- 
sally adopted  in  butter-making,  called  ripening,  or,  in  Europe, 
more  commonly  called  souring.  In  butter-making,  the  cream  is 
not  usually  churned  immediately  after  it  is  separated  from  the  milk, 
but  it  is  allowed  to  lie  in  a  moderately  warm  vat  for  a  period  of  twelve 
to  twenty-four  hours  or  even  longer,  that  it  may  ripen.  In  some 
places  there  is  a  demand  for  what  is  known  as  sweet  cream  butter, 
which  is  simply  butter  made  from  fresh  cream  without  ripening; 
but  such  a  demand  is  very  limited,  and  most  butter  is  made  from 
ripened  cream. 

CREAM-RIPENING. 

The  custom  of  ripening  cream  is  an  old  one,  doubtless  as  old 
as  the  process  of  butter-making.  Upon  a  farm  where  the  amount 
of  cream  is  small,  it  is  always  necessary  to  allow  it  to  accumulate 
for  some  days  till  there  is  sufficient  for  a  proper  churning.  During 
this  period  it  is  sure  to  undergo  ripening  without  any  intention  on 
the  part  of  the  farmer.  On  ordinary  farms,  the  cream  is  left  to  take 
care  of  itself,  and  is  thus  sure  to  be  ripened  by  the  time  there  is 
enough  to  churn.  But  the  centralization  of  butter-making  into 
creameries,  where  large  quantities  of  cream  are  handled  daily,  has 
put  a  new  aspect  upon  the  problem.  The  ripening  will  no  longer 
care  for  itself,  but  must  be  carefully  attended  to  by  the  butter-maker. 
The  necessity  for  some  accurate  means  of  controlling  the  ripening 

i8i 


l82  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

has  become  more  and  more  apparent  with  each  step  toward  the 
concentration  of  butter-making.  The  farmer  may,  perhaps, 
allow  his  cream  to  care  for  itself,  since  his  product  is  so  small. 
But  such  a  plan  would  ruin  a  creamery  where  there  are  thousands 
of  pounds  of  butter  made  each  day.  Only  as  the  ripening  can  be 
controlled,  is  concentration  of  butter-making  successful. 

The   Purposes   of   Cream-ripening. — These   are  as  follows: 

1.  Ripening  the  cream  makes  it  churn  more  easily  and  increases 
the  yield  of  butter.  This  is  true,  at  all  events,  for  gravity  cream; 
it  is  less  significant,  and  perhaps  not  true,  for  separator  cream. 

2.  Butter  made  from  properly  ripened  cream  is  thought  to  keep 
better.  3.  By  far  the  most  important  purpose  in  cream-ripening 
is  the  production  in  the  butter  of  a  desirable  flavor  and  aroma. 
Butter  made  from  unripened  cream  lacks  the  peculiar  flavor  of 
high-grade  butter,  since  this  is  the  result  of  the  ripening.  If  the 
ripening  is  not  satisfactory,  the  flavor  and  aroma  of  the  butter  are 
sure  to  be  inferior. 

The  importance  of  this  factor  in  butter-making  for  our  creameries 
is  very  great.  The  market  price  of  butter  depends  largely  upon  the 
flavor.  Butter  without  flavor  or  with  bad  flavor  brings  a  price 
in  the  market  which  hardly  pays  for  the  making,  while  a  product 
with  a  good  flavor  and  aroma  will  sell  for  at  least  three  or  four  cents 
more  a  pound;  and  the  exceptionally  fine-flavored  product  of  special 
creameries  brings  a  fancy  price — two  or  three  times  that  of  poor  butter. 
The  flavor  will  frequently  add  one-third  or  one-half  to  the  price 
which  could  be  obtained  for  poorly  flavored  butter  or  for  butter 
without  flavor.  Hence,  the  success  or  failure  of  a  creamery  business 
depends,  in  large  measure,  upon  the  ripening.  A  creamery 
which  fails  to  ripen  its  cream  properly  fails  to  obtain  a  desirable 
flavor.  Hence,  it  obtains  a  lower  price  for  its  butter  and  may 
hardly  meet  expenses;  while  a  neighboring  creamery,  that  is  more 
successful  in  its  cream-ripening,  obtains  a  good  product  and, 
consequently,  a  price  for  its  butter  which  makes  the  business  a 
financial  success.  This  matter  is  of  more  significance  to-day  than 
in  earlier  years,  because  our  butter-making  is  coming  to  be  concen- 
trated in  large  creameries. 


BACTERIA    IN    BUTTER -MAKING.  1 83 

The  Cause  of  Cream-ripening. — The  ripening  of  cream  is  a 
phenomenon  of  bacteria  growth.  The  many  bacteria  in  the  cream 
find  it  an  excellent  medium  for  food,  and  if  kept  at  a  fairly 
warm  temperature  during  the  ripening  period,  their  development 
is  rapid.  For  the  twelve  to  twenty-four  hours  of  ripening,  the 
bacteria  multiply,  and,  by  the  time  the  cream  is  ripened  and  ready 
to  be  churned,  they  are  present  in  prodigious  numbers.  Analyses 
of  ripened  cream  have  disclosed  the  fact  that,  whereas  in  the  sweet 
cream  bacteria  may  be  from  2,000,000  to  3,000,000  per  c.c,  in 
the  same  cream  when  ready  to  churn  there  may  be  about  500,000,000 
per  c.c.  The  numbers  at  the  time  of  ripening,  however,  vary  widely, 
being  sometimes  as  low  as  200,000,000,  or  even  lower,  and  sometimes 
as  high  as  2,000,000,000  per  c.c. 

The  growth  of  bacteria  in  the  cream  produces  chemical  changes 
which  considerably  modify  its  nature.  The  lactic  acid  bacteria 
always  develop  lactic  acid,  and  the  cream  becomes  sour;  but  there 
are  other  changes  as  well.  We  do  not  yet  know  what  all  these 
changes  are  or  to  what  extent  they  contribute  to  the  ripening 
phenomenon.  That  the  other  changes  have  something  to  do  with 
the  production  of  the  flavor  in  butter  is  evident  from  the  fact  that 
a  butter  flavor  cannot  be  produced  in  the  cream  by  adding  lactic 
acid  to  it,  and  if  the  ripening  were  wholly  the  result  of  souring, 
the  addition  of  lactic  acid  should  produce  the  same  results  as  normal 
ripening. 

Growth  of  Bacteria  During  the  Ripening. — At  the  outset 
cream  contains  many  kinds  of  bacteria,  and  the  composite  cream 
of  a  creamery  has  more  kinds  than  that  of  a  private  dairy.  The  cream 
is  commonly  kept  between  60°  and  70°,  at  which  temperature 
many  bacteria  develop  rapidly,  but  not  all  kinds  with  equal  vigor. 
During  the  first  few  hours  there  is  a  general  increase  in  the  number 
of  nearly  all  the  kinds  of  bacteria  originally  present  in  the  cream, 
so  that,  after  six  or  eight  hours,  there  are  higher  numbers  of  all 
species  of  bacteria  than  were  found  at  first.  During  this  time, 
however,  the  lactic  acid  bacteria,  especially  of  the  Bad.  acidi  lactici 
type,  increase  more  rapidly  than  the  others.  In  the  very  fresh 
cream  this  species  may  have  been  comparatively  small  in  numbers, 


184  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

forming  not  more  than  i  or  2  per  cent,  of  the  whole,  but  the  per- 
centage rises  rapidly.  After  several  hours,  the  time  varying  with 
different  specimens,  the  acid  bacteria  constitute  a  large  proportion 
of  the  whole.  From  this  time,  after  they  form  perhaps  50  per  cent, 
of  all  the  bacteria  present,  the  other  species  begin  to  be  seriously 
affected  by  the  acid  produced.  The  acid-forming  germs  still  con- 
tinue to  increase  in  numbers,  while  the  others  cease  to  grow  so 
rapidly,  soon  begin  to  diminish,  and  finally  may  largely  or 
wholly  disappear.  The  result  of  this  is  that,  during  the  last  stages 
of  the  ripening,  there  may  be  present  in  the  cream  nothing  but  acid 
bacteria,  which  sour  the  cream  and  produce  the  final  changes  in  the 
ripening. 

Thus  it  will  be  seen  that  the  ripening  of  cream  may  be  divided 
into  two  stages.  In  the  first  the  growth  of  the  miscellaneous 
species  of  bacteria  continues,  and  all  types  may  become  more  or  less 
abundant.  In  the  second  the  acid-forming  germs  gradually  force 
the  others  into  the  background  and  finally  crowd  them  out  entirely. 
Both  of  these  stages  doubtless  contribute  to  the  final  product. 
Without  the  proper  lactic  organisms  it  is  impossible  to  get  the 
proper  flavored  butter.  But  butter  made  from  pasteurized  cream 
and  ripened  by  pure  cultures  of  lactic  acid  bacteria  does  not  develop 
so  much  flavor  as  that  in  which  the  original  bacteria  are  allowed 
to  grow  with  the  acid  germs.  Hence,  it  is  probable  that  the  develop- 
ment of  the  miscellaneous  bacteria  in  the  first  phase  of  the  ripening 
has  not  a  little  to  do  with  the  final  butter  flavors. 

The  Effect  of  Different  Species  of  Bacteria. — The  butter- 
maker  thus  needs  bacteria,  but  he  must  have  the  right  kind.  When 
cream  is  collected  for  a  large  creamery  from  many  sources  there 
are  sure  to  be  in  it  quantities  of  different  varieties  of  bacteria,  each 
patron  contributing  his  quota.  Each  species  may  be  expected  to 
have  its  effect  upon  the  cream  during  the  ripening,  and  the  resulting 
butter  will  show  this  effect.  Actual  study  has  proved  that  different 
species  of  bacteria,  when  allowed  to  grow  in  the  ripening  cream, 
produce  very  different  types  of  butter.  Some  produce  hitter  butter, 
others  tainted  butter,  others  insipid  butter,  and  others  a  strong  odor, 
almost  like  that  of  putrefaction.     Some  species  produce  a  tallowy 


BACTERIA    IN    BUTTER-MAKING.  1 85 

butter,  others  a  turnip-tasting,  or  putrid  butter.  In  general,  it  is  the 
lactic  bacteria  which  produce  the  desired  results,  while  other  types, 
if  excessively  abundant,  give  rise  to  the  abnormal  flavors. 

Since  the  bacteria  are  so  varied  in  their  action,  it  may  be  a  matter 
of  surprise  that  cream-ripening,  if  left  to  itself,  so  commonly  results 
favorably.  The  primary  reason  for  this  is  the  superior  vigor  of  the 
lactic  acid  bacteria.  Since,  in  the  ordinary  bacterial  growth  in 
cream,  the  lactic  bacteria  finally  get  the  upper  hand  and  grow  at  the 
expense  of  all  the  others,  it  ordinarily  happens  that  the  ripening 
produces  a  good  flavor,  and  a  satisfactory  butter  is  obtained.  Un- 
fortunately, however,  the  favorable  species  of  lactic  bacteria  do  not 
always  get  the  upper  hand  in  the  cream-ripening.  Sometimes  large 
numbers  of  other  bacteria  are  present  in  the  cream,  just  as  vigorous 
and  just  as  capable  of  rapid  growth  as  the  desirable  lactic  acid 
germs.  In  such  cases  the  unusual  bacteria  may  develop  abundantly 
and  produce  a  variety  of  uncommon  changes  in  the  cream,  with  the 
result  of  giving  an  undesirable  flavor  to  the  butter.  Such  a  phe- 
nomenon explains  the  occasional  appearance  of  bad-tasting  butter. 
The  fact  that  such  improper  ripening  does  sometimes  occur  clearly 
points  to  the  need  of  some  control  over  the  ripening,  especially  in 
creameries  where  a  uniformly  good  product  is  necessary  for  financial 
success. 

CONTROL    OF   CREAM-RIPENING. 

The  butter-maker  has  no  control  over  the  kinds  of  bacteria  that 
get  into  his  cream,  and  a  creamery  must  take  cream  filled  with 
whatever  bacteria  chance  to  be  most  common  in  the  dairy  furnishing 
it.  But  though  he  cannot  control  this  factor,  he  can,  more  or  less 
satisfactorily,  regulate  the  growth  of  the  bacteria. 

Temperature  of  Ripening. — At  a  temperature  of  from  65°  to 
70°  the  favorable  lactic  acid  bacteria  get  the  upper  hand  of  other 
species  more  readily  than  at  either  a  higher  or  a  lower  temperature. 
At  temperatures  above  or  below  this,  different  species,  mostly  un- 
favorable, are  more  likely  to  gain  the  upper  hand.  Hence,  by 
keeping  the  temperature  at  about  65°,  the  undue  development  of 
mischievious  bacteria  is  more  likely  to  be  prevented. 
16 


1 86  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

Duration  of  Ripening.— The  butter-maker  can  stop  the 
ripening  at  any  point,  for,  after  the  cream  is  churned  into  butter, 
the  bacteria  growth  ce^-ses.  The  necessary  duration  of  the  ripen- 
ing will  vary,  however,  with  the  conditions.  Sometimes  cream, 
when  brought  to  a  creamery,  is  already  sour  and  has,  therefore, 
become  ripened  even  before  the  butter-maker  receives  it.  In  other 
cases,  especially  in  winter,  it  will  not  only  be  sweet,  but  will  contain 
small  numbers  of  bacteria  and  require  a  much  longer  ripening. 
Moreover,  milk  produced  under  good  dairy  conditions,  clean  and 
fairly  free  from  bacteria,  will  ordinarily  require  longer  ripening  than 
milk  produced  under  less  favoiable  conditions  and  containing  al- 
ready great  numbers  of  bacteria.  The  length  of  time  will  vary 
also  with  the  temperature,  being,  of  course,  longer  at  lower  tempera- 
tures. To  determine  when  the  cream  is  sufficiently  ripened,  the 
butter-maker  has  two  methods.  One  is  the  general  appearance  to 
his  eye  and  taste,  and  the  other  is  the  degree  of  acidity.  The  latter 
factor  is  determined  by  the  methods  described  on  page  309,  and 
the  ripening  is  generally  continued  until  the  acidity  is  0.5  to  0.65 
per  cent. 

THE  USE  OF  STARTERS. 

By  far  the  most  important  change  in  the  methods  of  cream- 
ripening  is  in  the  wide  and  almost  universal  introduction  of  starters. 
Twenty-five  years  ago  it  was  sometimes  customary  to  add  a  starter 
to  cream  in  cold  weather  simply  for  the  purpose  of  starting  the 
ripening;  but  to-day  almost  all  good  creameries  use  starters,  not  so 
much  for  starting,  as  for  regulating  the  ripening. 

Prof.  Storch,  of  Copenhagen,  first  conceived  the  possibility 
of  furnishing  to  butter-makers  cultures  of  the  proper  species  of 
bacteria,  which  they  might  add  to'  their  cream  for  the  purpose  of 
ripening,  somewhat  as  yeast  is  used  in  brewing.  This  experimenter 
not  only  conceived  the  method,  but  put  it  into  practical  operation 
in  Denmark.  His  method  consisted  i.  in  pasteurizing  the  cream 
at  about  165°  F.,  for  the  purpose  of  destroying  most  of  the  bacteria 
that  might  be  present,  and  2.  in  adding  to  it  a  pure  culture  of 
bacteria,  whose  value  in  producing  a  good  flavor  had  been  deter- 


BACTERIA    IN    BUTTER-MAKING.  187 

mined  by  experiment.  This  method  is,  of  course,  logically 
satisfactory,  for,  since  pasteurization^destroys  most  of  the  bacteria 
present  in  the  cream,  it  follows  that  the  ripening  will  be  produced 
by  the  species  of  bacteria  introduced  by  the  adding  of  the  pure 
culture.  Professor  Storch  was  soon  followed  by  other  experimenters 
and  the  method  adopted  in  Copenhagen  was  extended  more  or  less 
widely  in  north  Germany  and  Denmark.  In  Denmark  it  is  now 
used  almost  universally,  and  in  north  Germany  quite  widely,  in 
general  dairying. 

In  the  United  States  the  use  of  pure  cultures  for  cream-ripening 
has  had  a  somewhat  different  history.  It  was  introduced  to  dairy- 
men shortly  after  its  development  in  Copenhagen,  but  for  some 
time  little  attention  was  paid  to  it,  so  that  it  was  hardly  brought  to 
the  notice  of  the  ordinary  butter-maker.  Our  butter-makers  were 
not  in  condition  to  pasteurize  their  cream.  In  1895  a  slight  change 
was  made  in  the  process.  In  order  to  bring  the  subject  more 
widely  to  the  attention  of  dairymen,  a  method  was  suggested  of 
using  the  cultures  without  previously  pasteurizing  the  cream. 
This  seemed  illogical,  since  the  cream  is  already  filled  with  bacteria, 
and  the  addition  of  a  new  culture  could  hardly  be  supposed  to  give 
entirely  satisfactory  results.  But  when  we  remember  how  a  vigorous 
lot  of  lactic  acid  bacteria  can  overcome  other  species,- the  method 
does  not  appear  so  illogical  after  all.  With  this  change  our  butter- 
makers  were  willing  to  try  pure  cultures  and  in  a  short  time  American 
butter-makers  learned  of  their  meaning  and  began  to  experiment 
with  them  widely.  The  result  of  the  dozen  or  so  years  of  experience 
has  been  to  show  the  extreme  value  of  starters  as  a  means  of  controll- 
ing the  ripening,  until  to-day  starters  of  some  kind  are  almost  univer- 
sally used  in  all  good  creameries  and  dairies. 

Preparation  of  Starters.— While  starters  are  very  widely 
used  to-day,  they  are  not  always  pure  cultures.  Two  quite  different 
methods  of  preparing  them  are  in  use. 

Natural  Starters. — A  natural  starter  is  nothing  more  than  some 
normally  soured  milk.  In  order  to  obtain  it  it  is  only  necessary 
to  select  several  quarts  of  good  milk  and  place  it  in  a  clean,  sterilized 
pail  or  can,  covered  to  keep  out  the  dust,  and  keep  it  in  a  temperature 


1 88  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

of  from  65°  to  70°.  After  one  or  two  days  the  milk  should  show 
signs  of  souring;  when  it  has  become  decidedly  sour,  but  not  yet 
curdled,  it  is  to  be  used  as  a  starter.  It  requires  some  skill  on  the 
part  of  the  butter-maker  to  know  whether  the  starter  thus  obtained 
is  of  the  best  character  and  whether  it  should  be  used  or  thrown 
away  and  another  obtained.  Starters  made  in  this  way  are  not 
sure  to  be  uniform,  inasmuch  as  the  different  samples  of  milk  may 
contain  different  types  of  bacteria,  and  experience  is  needed  on  the 
part  of  the  butter-maker  to  know  whether  the  starter  is  satisfactory. 

Starters  from  Commercial  Cultures. — Commercial  starters  are 
now  a  well-known  article,  and  several  different  brands  may  be  pur- 
chased. In  all  cases  they  are  prepared  by  bacteriologists  and 
consist  of  a  culture  of  bacteria — usually  a  pure  culture,  though  not 
always — that  have  been  found  by  experiment  to  produce  favorable 
results  in  the  ripening.  These  starters  as  purchased  are  sometimes 
in  the  form  of  a  powder,  sometimes  in  the  form  of  a  liquid,  but  in 
all  cases  contain  too  small  a  quantity  to  add  directly  to  the  cream 
that  is  to  be  ripened.  The  quantity  of  bacteria  must,  therefore, 
be  increased  before  using  by  a  process  called  building  up.  The 
procedure  is  as  follows: 

A  quart  of  skim  milk,  whole  milk,  or  cream  is  placed  in  a  glass 
jar  and  sterilized,  either  by  boiling  or,  better,  by  pasteurizing  at 
180°  for  half  an  hour,  stirring  frequently  to  insure  uniform  heating. 
The  milk  is  then  cooled,  and  when  it  has  reached  a  temperature  of 
80°  the  commercial  culture,  from  Si  freshly  opened  package,  is  thor- 
oughly stirred  in;  the  whole  is  covered  to  keep  out  the  dust  and 
placed  at  a  temperature  of  about  65°.  When  the  milk  has  become 
quite  sour,  but  before  it  is  curdled,  it  is  ready  to  use  as  a  starter. 
If  a  larger  amount  of  starter  is  needed,  this  first  starter  is  placed  in 
a  large  can  of  pasteurized  milk  and  allowed  to  grow  in  it  at  65° 
until  the  whole  becomes  soured.  By  this  means  any  desired  amount 
can  be  prepared. 

The  starter  thus  prepared  is  added  to  the  cream  in  varying 
proportions,  the  larger  the  amount  the  quicker  the  ripening.  Some- 
times one  part  of  the  starter  to  ten  parts  of  cream  is  used;  in  other 
cases  a  smaller  amount  is  used  and  sometimes  more.     After  the 


BACTERIA    IN    BUTTER-MAKING.  1 89 

ripened  cream  is  ready  tb  churn,  a  certain  quantity  of  it  is  removed, 
placed  in  a  clean  can,  and  set  aside  to  serve  as  a  starter  for  the  next 
day's  churning.  In  this  way  some  starter  is  reserved  each  day, 
to  be  used  in  the  cream  collected  that  day;  and  thus  the  original 
starter  is  carried  on  from  churning  to  churning.  After  some  days, 
however,  it  is  necessary  to  resort  once  more  to  a  pure  culture,  built 
up  in  the  same  way. 

There  is  not  very  much  to  choose  between  natural  starters  and 
commercial  cultures.  Natural  starters  cost  nothing  except  the 
trouble  of  making  them,  but,  on  the  other  hand,  they  are  not 
uniform,  and  not  always  to  be  depended  upon.  Commercial  cultures 
cost  a  small  sum,  but  they  are  rather  more  uniform  than  natural 
starters.  It  has  been  claimed  that  the  flavor  of  butter  from  cream 
ripened  with  a  natural  starter  is  higher  than  that  ripened  with  a  pure 
culture.  This  is  easy  to  understand.  A  good  starter  should  sour 
cream  promptly;  should  thrive  at  60°  to  72°;  should  coagulate 
milk  and  cream  into  a  homogeneous  mixture,  and  should  produce 
an  agreeable  aromatic  taste.  No  single  bacterium  known  has  all 
these  characteristics,  but  a  mixture,  such  as  a  natural  starter,  may 
have  them.  On  the  other  hand,  if  a  creamery  notices  the  develop- 
ment of  "off  tastes"  in  the  butter,  the  best  method  of  removing  them 
is  by  the  use  of  a  commercial  pure  culture.  Both  kinds  of  starters 
thus  have  their  advantages. 

THE  USE  OF  STARTERS. 

In  Pasteurized  Cream. — If  the  cream  is  first  pasteurized  so  as 
to  destroy  most  of  the  bacteria  present,  the  added  starter  will  have  a 
free  chance  to  grow.  The  pasteurizing  of  cream  is  simple  and  not 
very  expensive,  and  it  produces  a  medium  largely  free  from  bacteria. 
The  use  of  starters  in  pasteurized  cream  has  become  practically 
universal  in  Denmark  and  some  of  the  other  countries  of  Northern 
Europe.  There  are  two  reasons  for  this:  i.  A  higher  and  more 
uniform  grade  of  butter  can  be  obtained  in  this  way.  2.  The 
prevalence  of  tuberculosis  has  brought  about  the  enactment  of  a  law 
requiring  all  milk  that  goes  through  the  creamery  to  be  pasteurized 
in  order  to  destroy  the  tuberculosis  germs.    For  this  reason  Den- 


IQO  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

mark  butter  is  always  made  from  pasteurized  cream,  and  this 
makes  it  necessary  to  use  an  artificial  starter,  since  pasteurized 
cream  will  not  ripen  of  itself.  The  pasteurization  destroys  practi- 
cally all  of  the  acid  bacteria,  and,  as  we  have  learned,  when  the  acid 
bacteria  are  absent  the  putrefying  bacteria  are  quite  sure  to  develop. 
Hence,  pasteurized  milk  requires  an  acid  starter  to  insure  a  proper 
ripening. 

In  Unpasteurized  Cream.— By  this  method  the  starter  is 
simply  added  to  the  ordinary  cream.  The  use  of  starteis  in  this 
way  is  open  to  a  theoretical  objection.  The  cream  already  con- 
tains bacteria  in  large  numbers  and,  ordinarily,  in  considerable 
variety.  These  would  themselves  produce  the  ripening  of  cream, 
even  without  any  starter.  The  effect  of  the  starter  added  to  the 
cream  already  filled  with  bacteria  will,  evidently,  not  always  be 
uniform.  It  might  produce  little  or  no  effect,  or,  if  the  starter  is 
added  in  considerable  quantity,  it  might  overcome  the  effect  of  the 
smaller  number  of  bacteria  originally  in  the  cream.  In  practice  it  is 
found  that  the  use  of  starters  does  have  this  latter  effect,  and  in  most 
cases,  there  is  a  noticeable  improvement  in  butter  made  from  cream 
thus  ripened.  The  results,  however,  are  not  absolutely  uniform, 
and  even  with  the  use  of  a  large  amount  of  starter  it  will  sometimes 
happen  that  the  bacteria  present  in  the  cream  will  have  more  in- 
fluence than  those  of  the  starter,  and  the  butter  will  suffer. 

The  use  of  cultures  in  unpasteurized  cream  was  first  begun  in  the 
United  States  and  has  been  more  widely  adopted  here  than  any- 
where else.  Butter  made  from  unpasteurized  cream  is  not  so 
uniform  as  that  made  from  pasteurized  cream,  but  the  butter  made 
in  this  way  is,  at  least  to  the  American  taste,  superior  to  butter  made 
with  pasteurization,  due  probably  to  the  fact  that  pasteurization 
prevents  the  growth  of  miscellaneous  bacteria  that  ordinarily 
occurs  before  the  lactic  bacteria  develop.  Pasteurized  cream  butter 
is  somewhat  milder  in  flavor  than  that  made  from  unpasteurized 
cream,  and  the  American  market  demands  a  flavor  somewhat 
stronger  than  that  which  is  popular  in  Europe.  Hence,  to  the 
American  taste,  up  to  the  present  time,  the  butter  from  pasteurized 
cream  is  not  superior  to  that  made  from  unpasteurized  cream. 


BACTERIA    IN    BUTTER-MAKING.  19I 

»«  . 
THE  GENERAL  VALUE  OF  STARTERS. 

The  fact  that  starters,  with  or  without  pasteurization,  have 
become  almost  universally  used  among  the  better  class  of  cream- 
eries is  in  itself  sufficient  proof  that  they  are  of  practical  value. 
Their  advantage  lies  in  four  directions:  1.  They  enable  the  butter- 
maker  to  handle  his  cream  more  easily  and  uniformly.  He  can 
regulate  the  ripening  in  such  a  way  that  his  cream  will  always  be  of 
a  certain  grade  of  ripeness  at  a  certain  time  of  day;  for  a  little 
experience  tells  him  how  much  of  his  culture,  under  proper  condi- 
tions, should  be  added  to  the  cream  to  produce  the  proper  grade  of 
ripening  at  the  particular  time  when  he  desires  to  churn.  2.  The 
use  of  starters  has  produced  a  greater  uniformity  in  the  grade  of 
butter.  The  butter-maker  can  depend  more  certainly  upon  pro- 
ducing butter  of  a  high  grade,  month  after  month,  than  he  can  with- 
out starters.  There  is  a  general  belief  also  among  those  who  have 
tested  the  butter  in  countries  where  starters  are  widely  used,  that 
there  is  an  improvement  in  the  average  quality  of  the  butter,  as  well 
as  in  its  uniformity.  3.  It  has  become  pretty  definitely  agreed  that 
the  flavor  of  butter  is  improved  by  the  use  of  such  cultures.  It  is 
somewhat  difficult  to  obtain  definite  proof  of  this,  owing  to  the  un- 
certainty of  scores  in  butter  tests.  But  the  fact  that  all  good  dairies 
now  use  them  is  sufficient  testimony  to  their  value  in  improving  the 
general  quality  of  the  butter.  4.  They  are  the  best  means  of 
remedying  butter  "faults."  Every  creamery  has  experiences  of 
deterioration  in  the  flavor  of  the  butter  without  any  visible  cause. 
Such  troubles  are  known  to  be  due  commonly  to  the  growth  of 
unusual  and  undesirable  bacteria  in  the  cream.  When  they  are 
discovered,  the  sterilizing  of  the  dairy  utensils  and  the  use  of  a  large 
quantity  of  a  vigorous  starter  will  generally  remedy  the  trouble  at 
once.  Moreover,  the  constant  use  of  a  starter  goes  a  long  way 
toward  preventing  these  "faults." 

It  is  doubtful  whether  the  use  of  starters  produces  butter  of  a 
character  superior  to  the  best  butter  made  without  them.  Indeed, 
some  think  that  it  is  not  quite  equal  to  the  best  butter  made  without 
starters.     But  the  uniformly  high  grade  of  culture  butter  is  admitted, 


192  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

and  the  greater  satisfaction  in  being  able  to  control  the  process  has 
caused  the  wide  adoption  of  starters  among  butter-makers. 

BACTERIA  IN  BUTTER. 

Although  bacteria  continue  to  grow  during  the  ripening  period, 
their  growth  is  practically  stopped  by  churning  and  butter-making. 
Many  of  them  are  removed  with  the  butter-milk;  others  are  washed 
away  during  the  washing  and  working  of  the  butter.  Large 
numbers  are  still  left  in  the  butter.  Ordinarily  these  bacteria  do  not 
grow  in  the  butter,  though,  if  it  is  not  salted,  some  of  them  may 
grow  and  hasten  its  spoiling.  Unsalted  butter  does  not  keep  long, 
and  its  destruction  is  largely  due  to  bacteria.  But  if  the  butter  is 
salted,  as  is  the  rule  in  most  countries,  the  salt  checks  the  growth  of 
bacteria.  As  a  result  of  this  and  the  compact  condition  of  the 
butter,  together  with  its  small  amount  of  water,  the  bacteria  do  not 
find  butter  a  favorable  medium  for  growth,  and  they  begin  to  diminish 
in  numbers.  A  very  few  hours'  time  shows  a  great  reduction  in 
numbers,  and  this  continues  until,  after  a  few  weeks,  the  butter  con- 
tains comparatively  few.  They  do  not  entirely  disappear,  for  some 
are  found  even  in  very  old  butter.  The  ordinary  species  of  organ- 
isms, which  have  been  active  agents  in  the  cream-ripening,  play  no 
further  part  in  the  changes  which  may  occur  in  the  butter.  The 
following  figures  of  the  number  of  bacteria  in  butter  will  illustrate 
the  facts. 

No.  of  Bacteria  per  Gram  of  Butter. 
Two  hours  old.     One  day  old.     Four  days  old.     Thirty  days  old. 
■  54,000,000  26,006,000  2,000,000  300,000. 

It  is  well  known  that,  if  butter  is  not  used  immediately,  certain 
changes  occur  in  it  which  continue  slowly  for  many  weeks.  The 
butter  retains  its  fresh,  delicate  flavor  and  aroma  for  only  a  few 
days;  but  if  it  is  kept  cool  and  away  from  the  light,  it  may  remain 
sweet  and  good  for  many  months.  If,  however,  it  is  not  kept  very 
cold,  further  changes  soon  begin  to  appear  which  slowly  progress 
and  eventually  ruin  the  butter.  The  most  noticeable  feature 
is  the  appearance  of  rancidity.     This  change  is  accompanied  by  the 


BACTERIA    IN    OLEOMARGINE    PRODUCTS.  1 93 

development  of  butyric  acid  and  frequently  by  a  considerable 
change  in  the  C(^nsistency  of  the  butter.  It  finally  becomes  strongly 
rancid  and  tallowy,  totally  ruined  for  use.  The  cause  of  this 
rancidity  has  been  difficult  to  determine,  apparently  because  a 
variety  of  factors  contribute  to  it.  It  is  probably  due,  in  part,  to 
chemical  fermentation,  produced  by  enzymes  in  the  milk,  and  in 
part  to  the  growth  of  bacteria.  The  rancidity  is  much  more  likely 
to  occur  if  the  butter  is  exposed  to  the  light,  and  it  develops  more 
readily  in  warm  than  in  cold  temperatures.  At  temperatures  below 
freezing  rancidity  does  not  occur.  If  butter  is,  therefore,  kept 
cool  and  in  large  masses,  it  may  be  held  for  a  long  time  without 
the  appearance  of  any  very  noticeably  strong  flavor.  In  the  end, 
however,  the  rancidity  is  sure  to  appear.  To  what  extent  bacteria 
are  concerned  in  this  change  we  do  not  yet  know,  although  most 
investigators  have  concluded  that  they  are  prominently  concerned 
in  the  phenomenon.  Rancidity  may  certainly  be  looked  upon  as 
a  fermentation  change,  and  the  only  method  the  dairyman  has  of 
controlling  it  is  by  cool  temperatures,  by  packing  the  butter  in  large 
masses,  by  paraffining  the  butter  tubs,  and  by  keeping  it  from  the 
light.  It  may  be  delayed  by  pasteurizing  the  cream  and  by  using 
pasteurized  water  for  washing,  facts  that  show  its  close  relation  to 
bacteria.  Fortunately,  it  is  a  matter  of  no  very  great  importance, 
because  butter  can  be  kept  without  difficulty  for  some  months,  and 
it  is  almost  always  possible  to  market  it  before  it  has  spoiled. 

BACTERIA  IN  OLEOMARGARINE  PRODUCTS. 

The  materials  out  of  which  oleomargarine  is  m^de  are  chiefly 
stearin,  lard,  cottonseed  oil,  and  other  oils.  These  are  warmed  to 
the  melting-point,  are  thoroughly  mixed,  and  then  drawn  off  into 
cold  brine,  which  chills  the  oils  into  a  hard  mass.  The  process 
is  certainly  a  useful  method  of  utilizing  quantities  of  oils  which 
would  otherwise  be  waste  products.  It  makes  a  -wholesome, 
digestible  food,  which  could  have  no  objection  raised  against  it 
if  it  could  only  be  sold  upon  its  own  merits,  instead  of  under  the 
false  guise  of  butter.  In  order  to  make  the  product  the  more 
resemble  butter,  it  has  been  customary  to  color  it;  but  this  practice 
17 


194  BACTERIA    IN    BUTTER   AND    OLEOMARGARINE. 

is  now  largely  prevented  by  a  prohibitive  tax  on  colored  oleomarga- 
rine. The  bacteria  normally  present  in  oleo  products  are  commonly 
much  less  numerous  than  in  butter,  and  the  oleomargarine  is,  on 
the  whole,  less  likely  to  distribute  infectious  diseases  than  ordinary 
butter,  inasmuch  as  the  chance  for  contamination  is  less. 

But,  although  the  oleo  products  thus  made  resemble  butter  in 
appearance,  they  do  not  resemble  it  in  taste,  and  the  factories  are 
therefore  forced  to  use  some  special  method  of  imparting  to  their 
product  a  flavor  as  closely  as  possible  like  the  butter  which  they  are 
trying  to  imitate.  To  do  this  they  depend  upon  the  very  same  flavors 
as  those  found  in  butter  and  obtain  them  from  a  similar  source. 
A  certain  amount  of  whole  milk,  skim  milk,  or  cream  (varying 
according  to  the  quality  desired  in  the  product)  is  placed  in  a  large 
vat,  or  in  cans,  and  allowed  to  sour.  After  the  milk  has  properly 
soured,  or  ripened,  it  is  placed  in  the  mixing  vat  with  a  quantity  of 
melted  oils,  generally  in  the  proportion  of  about  one  part  of  milk  to 
four  parts  of  the  oils.  When  this  mixture  is  hardened  by  the  cold 
brine,  the  milk  is  held  with  fats,  and  thus  becomes  a  part  of  the 
final  product.  Inasmuch  as  the  milk  has  developed  a  flavor  in  its 
souring,  just  as  cream  does  during  its  ripening,  this  flavor  is  imparted 
to  the  oleo  product,  and  the  final  result  is  a  mass  of  fats  with  the 
flavor  of  butter  more  or  less  prominently  developed. 

It  is  clear  that  this  flavor  is  due  to  exactly  the  same  factors  as 
those  which  produce  the  butter  flavor.  The  oleo-maker  fully 
understands  that  his  flavors  are  due  to  the  action  of  bacteria,  and 
he  uses  the  best  means  at  his  disposal  to  favor  their  growth.  Ordi- 
narily he  aUows  his  milk  to  sour  by  normal  lactic  fermentation. 
In  some  factories,  in  recent  years,  he  has  not  been  satisfied  to  depend 
upon  such  a  method,  but  has  come  to  use,  more  and  more  largely, 
pure  cultures  of  bacteria  in  order  to  introduce  greater  regularity  in 
the  process.  In  some  oleo  factories,  indeed,  so  fully  aware  have  the 
makers  been  of  the  extreme  significance  of  this  matter  of  proper 
bacteria  to  the  successful  manufacture  of  oleo  products,  that  they 
have  actually  built  and  furnished  bacteriological  laboratories  and 
employed  bacteriologists  to  keep  constant  guard  over  these  factors 
in  the  oleo-making. 


CHAPTER  XIV. 

BACTERIA  AND  OTHER  MICROORGANISMS  IN 
CHEESE. 

CHEESE-RIPENING. 

Cheese  consists  primarily  of  the  casein  and  fat  of  milk,  collected 
first  as  a  curd  and  then  allowed  to  undergo  a  series  of  chemical 
changes  called  ripening.  Ordinarily  the  casein  is  precipitated 
from  the  milk  by  rennet,  although  it  is  done  in  some  types  of  cheese 
by  simple  souring.  Then  the  curd  is  separated  more  or  less  from 
the  whey,  and  pressed  into  definite  shape.  The  whey  removes 
most  of  the  milk-sugar,  and  the  cheese  retains  about  two-thirds 
of  the  food  material  in  the  milk,  and  since  it  is  in  a  very  dense  form, 
it  is  one  of  the  most  nutritious  of  our  foods.  The  popularity  of 
cheese  as  a  food  depends  rather  upon  its  flavor  than  its  food  value 
and  the  flavor  develops  during  the  ripening.  Cheese-ripening 
is  a  very  complex  phenomenon  and  one  as  yet  only  partly  understood. 
This  is  due  partly  to  the  intricacy  of  the  subject  and  partly  also  to 
the  fact  that  there  are  very  many  different  kinds  of  cheeses,  and  the 
ripening  of  the  different  types  is  not  by  any  means  the  same.  Al- 
though there  are  some  hundred  varieties  of  cheese,  they  may  be  ar- 
ranged fairly  well  into  two  groups:  i.  The  hard  cheeses,  and 
2.  The  soft  cheeses.  The  ripening  of  the  hard  cheeses  is  very  dif- 
ferent from  that  of  the  soft,  and  the  ripening  of  the  different  types 
of  soft  cheese  varies  greatly  one  from  the  other.  Each  kind  of 
cheese  must,  therefore,  be  studied  as  a  special  problem. 

Chemical  and  Physical  Changes. — During  the  ripening, 
the  cheese,  which  is  at  first  rather  hard,  tough  and  elastic,  gradually 
becomes  softer.  The  extent  of  this  softening  depends  largely 
upon  the  amount  of  water  present,  and,  in  the  soft  cheese,  with  their 
large  amount  of  water,  a  slimy  and  almost  liquid  consistency  may 

195 


196        BACTERIA  AND    OTHER   MICROORGANISMS    IN    CHEESE. 

finally  be  reached.  This  softening  is  due  to  a  change  in  the  chemical 
nature  of  the  casein  by  which  it  changes  from  an  insoluble  into  more 
or  less  soluble  products.  The  changes  in  the  casein  during  the 
ripening  are,  in  general,  similar  to  those  that  occur  when  the  casein 
is  digested  under  the  action  of  digestive  juices,  so  that  they  are 
frequently  spoken  of  as  a  digestion  of  the  curd.  During  the  ripening 
the  insoluble  casein  is  converted  into  a  series  of  simpler  chemical 
bodies,  peptones,  proteoses ,  etc.,  and  as  these  become  partly  dissolved 
the  hard  texture  of  the  cheese  becomes  softer.  Not  only  are  the 
changes  similar  to  those  of  digestion,  but  they  are  produced  by 
enzymes  similar  to,  though  probably  not  identical  with,  the  enzymes 
in  the  digestive  juices.  The  ripening  of  a  cheese  is  thus  a  prediges- 
tion  which  renders  the  cheese  more  easily  digested  when  eaten.    ^' 

The  enzymes  that  produce  this  ripening  of  the  cheese  come  froAi 
three  quite  different  sources.  One  enzyme  with  this  power  of 
digesting  curd  is  in  the  original  milk  itself.  This  is  the  galactas'e 
already  mentioned.  Another  is  added  to  the  milk  with  the  rennet', 
for  rennet  is  made  from  the  stomach  of  a  mammal,  and  will  always 
contain  some  of  the  pepsin  from  the  stomach.  The  latter  is  an 
enzyme  with  strong  digestive  power  and  is  sure  to  be  present  in 
some  quantity  in  the  rennet,  and  hence  in  the  cheese  after  the  addition 
of  rennet.  These  two  enzymes  doubtless  continue  to  act  upon  the 
casein  during  the  ripening,  and  are  responsible  for  a  certain  portion 
of  the  digestive  changes  that  are  taking  place.  But  there  is  also  a 
third  source  of  enzyme  that,  in  some  cheeses,  is  more  important  than 
the  others.  As  already  noticed,  certain  microorganisms  have  the 
power  of  secreting  enzymes,  and  some  of  them,  growing  in  or  on  the 
ripening  cheese,  develop  enzymes  which  contribute  largely  to  the 
ripening.  In  some  of  the  soft  cheeses  this  is  certainly  the  chief 
source  of  the  enzymes. 

Flavors. — The  production  of  flavors  is  of  no  less  importance 
than  the  chemical  digestion  of  the  cheese.  At  the  present  time, 
however,  there  is  a  very  profound  ignorance  concerning  the  real 
source  and  cause  of  cheese  flavors.  They  are  without  doubt  the 
products  of  decomposition.  They  appear  in  the  cheese  only 
toward  the  end  of  the  ripening  process,  and  are  regarded  generally 


THE    HARD    CHEESES.  I97 

as  due  to  the  end-products  of  decomposition.  The  peptones  and 
proteoses  that  result  from  the  digestion  of  the  caseins  do  not  them- 
selves have  any  of  these  peculiar  cheese  flavors;  but  toward  the  end 
of  the  ripening  some  of  the  material  seems  to  be  still  further  broken 
down  into  the  simpler  end-products  which  show  the  flavors  charac- 
teristic of  cheese. 

The  relation  of  microorganisms  to  the  ripening  of  hard  and  soft 
cheeses  is  so  different  that  they  must  be  considered  separately. 

THE  HARD  CHEESES. 

These  include  many  varieties,  the  best  known  of  which  are  the 
Cheddar  cheeses  (the  most  common  in  America),  the  Swiss 
cheeses,  and  the  Edam  cheeses  of  Holland.  In  all  these  cheeses  the 
water  is  pressed  out  of  the  curd  as  completely  as  possible  so  that  the 
resulting  cheese  is  very  dense.  In  spite  of  its  dense  consistency,  a 
development  of  bacteria  goes  on  for  many  days  during  the  ripening. 
Since  cheese  is  made  from  milk,  it  must  necessarily  contain  from 
the  outset  many  kinds  of  bacteria,  and  among  them  there  is  a  con- 
siderable percentage  of  lactic  acid  organisms.  For  a  number  of 
days,  sometimes  for  several  weeks,  these  bacteria  increase  in  number. 
After  this  they  decrease  until,  when  fully  ripened,  they  are  very  few, 
compared  to  their  numbers  at  certain  stages  of  the  ripening.  A 
single  example  will  illustrate.  Fresh  cheese  contained  6,600,000 
per  gram,  when  four  days  old,  51,000,000  per  gram,  and  when  four 
months  old,  1,000,000  per  gram. 

It  is  the  lactic  acid  bacteria  that  are  most  persistent,  and  while 
in  the  early  stages  of  the  ripening  other  types  are  abundant,  in  the 
fully  ripened  cheese,  or  even  the  half  ripened  cheese,  there  are  usually 
none  left  except  the  lactic  acid  bacteria.  While  there  are  variations 
in  the  bacteria  in  different  kinds  of  cheese  and  in  different  specimens 
of  the  same  variety,  the  above  represents  the  general  history  of  their 
growth,  and  in  all  cases  it  appears  that  the  lactic  acid  organisms  are 
finally  found  alone. 

It  must  be  confessed  that  we  do  not  yet  know  very  much  about 
the  part  bacteria  play  in  the  ripening  of  the  hard  cheeses.     That 


198        BACTERIA   AND    OTHER    MICROORGANISMS    IN    CHEESE. 

they  are  necessary  to  the  ripening  is  proved  by  the  fact  that  cheeses 
do  not  ripen  normally  when  they  are  ripened  in  chloroform  vapor, 
which  prevents  bacteria  growth,  but  allows  the  enzyme  action  to 
continue  as  usual.  Although  some  of  the  digestive  changes  go  on  as 
usual  in  these  cheeses,  the  ripening  does  not  become  complete, 
and  the  cheeses  never  develop  either  the  same  final  chemical  char- 
acter or  the  flavors  of  cheeses  in  which  the  bacteria  have  had  an 
opportunity  for  growth. 

Chemical  Action  of  Bacteria. — When  cheese-ripening  was 
first  studied,  it  was  believed  to  be  primarily  due  to  the  action  of 
bacteria.  We  have  already  seen  that  certain  kinds  of  bacteria  have 
the  power  of  changing  casein  to  peptone — the  liquefying  type — and 
this  change  in  the  cheese-ripening  was  at  first  supposed  to  be  due  to 
the  growth  of  these  peptonizing  bacteria.  But  later  it  became  evi- 
dent that  the  liquefying  bacteria  are  not  common  in  cheeses,  espe- 
cially in  the  better  grades.  If  present  at  the  beginning  they  rapidly 
decrease  in  numbers,  until  they  almost  or  entirely  disappear,  a  fact 
which  forced  the  conclusion  that  they  cannot  contribute  materially 
to  the  ripening  of  cheeses.  More  recently,  it  has  been  claimed  that 
certain  "acid  liquefiers" — i.e.,  peptonizing  bacteria  that  at  the  same 
time  produce  acid — are  intimately  connected  with  the  ripening. 
But  there  does  not  yet  appear  to  be  much  evidence  for  this. 

These  facts  led  to  a  suggestion  that  the  ripening  is  due  really  to 
the  lactic  acid  bacteria.  These  do  not  liquefy  gelatin  and  do  not 
ordinarily  have  any  power  of  changing  casein  to  peptone.  They 
produce  lactic  acid  which  curdles  the  milk,  after  which  they  ap- 
parently cease  to  act  upon  it  at  all.  Hence,  it  would  not  seem  that 
they  could  digest  cheese.  But  if  the  acid  which  they  produce  is 
neutralized  by  the  presence  of  some  alkaline,  like  carbonate  of  soda, 
the  bacteria  continue  to  grow,  and  eventually  produce  the  peptoniza- 
tion of  the  casein.  Moreover,  the  grade  of  the  cheeses  is  very 
closely  dependent  upon  the  growth  of  lactic  bacteria,  and  cheese 
from  which  lactic  acid  bacteria  are  excluded  by  aseptic  milking  will 
not  ripen  normally,  while  they  would  do  so  if  the  acid  germs  were 
present.  All  of  these  facts  together  led  to  the  conclusion  that  it  is 
this  peptonizing  power  of  the  lactic  acid  bacteria,  under  certain  con- 


THE    HARD    CHEESES. 


199 


ditions,  which  is  responsible  for  the  chemical  changes  that  take 
place  in  the  ripening  cheese.  This  conclusion,  however,  has  not 
been  very  generally  accepted;  for  while  the  lactic  acid  bacteria, 
under  these  conditions,  do  produce  a  certain  amount  of  peptoniza- 
tion of  the  casein,  the  action  is  extremely  slow  and  not  very  complete; 
and  it  has  not  seemed  to  most  students  that  the  phenomenon  in 
question  is  sufficiently  explained  by  this  slow  action  of  the  lactic  acid 
bacteria. 

Flavor  Production  by  Bacteria. — Apparently  the  jlavors  must 
be  due  to  bacterial  action.  Cheeses  ripened  in  chloroform  vapof, 
whicih  allows  the  enzymes  to  act,  but  prevents  bacteria  from  growing, 
though  they  ripen,  do  not  develop  flavors  and  these  must  be  due  to 
some  other  cause  than  enzyme  action.  That  they  are  the  end- 
product  of  chemical  decomposition  seems  to  be  extremely  probable. 
In  many  cases  they  are  associated  with  ammonia;  and  ammonia,  as 
is  well  known,  is  one  of  the  final  products  of  proteid  destruction. 
The  only  known  agency  that  commonly  produces  the  complete 
destruction  of  proteids  is  bacterial,  and,  while  the  matter  has  never 
been  put  to  any  satisfactory  test,  the  most  probable  explanation 
seems  to  be  that  these  cheese  flavors  are  the  result  of  bacterial 
decomposition. 

Against  this  view,  however,  has  been  urged  the  fact  that  in  the 
well  ripened  cheeses  hardly  any  bacteria,  except  lactic  acid  organisms, 
are  present,  and  that  this  class  of  bacteria  does  not,  so  far  as  is 
known,  have  any  power  of  producing  cheese  flavors.  Some 
bacteria,  if  they  grow  in  proper  abundance  in  milk,  will  in  time 
develop  well-known  cheese  flavors;  but  these  organisms  have  not 
been  found  in  old,  strongly  flavored  cheeses.  Whether  they  have 
anything  to  do  with  the  production  of  cheese  flavors  is,  therefore, 
uncertain.  It  has  been  suggested  that  the  flavor  of  cheeses  may  be 
due  to  the  bacteria  which  grow  in  them  during  the  first  few  days. 
Liquefying  bacteria  are  found  during  this  early  period,  and  before 
the  miscellaneous  bacteria  disappear,  as  they  do  later,  some  of  these 
liquefiers  may  secrete  from  their  bodies  substances,  possibly  enzymes, 
that  continue  their  action  in  the  cheese,  slowly,  but  for  a  long  time. 
Although  the  bacteria  that  produce  them  soon  die,  the  chemical 


200        BACTERIA   AND    OTHER    MICROORGANISMS    IN    CHEESE. 

ferments  which  they  have  produced  may  continue  their  activity  until 
they  finally  produce  the  new  products  that  give  the  flavor. 

One  fact  appears  to  be  certain  amid  much  that  is  still  unsettled. 
The  lactic  acid  organisms  certainly  play  an  important  part  in  the 
process;  at  least  in  the  ripening  of  the  Cheddar  cheeses  and  probably 
the  other  hard  cheeses  as  well.  The  lactic  acid  developed  in  the 
early  ripening  cheese  is  necessary  to  the  digestive  changes  that  occur, 
for  the  acid  combines  with  the  casein,  a  preliminary  step  in  the 
ripening.  While  their  total  action  is  not  yet  understood,  it  is 
certain  that  they  have  a  necessary  part  in  the  cheese-ripening. 

As  a  result  of  these  facts,  cheese-makers  have,  in  recent  years, 
learned  that  the  use  of  lactic  acid  starters  is  decidedly  advantageous. 
This  practice  enables  the  cheese-maker  to  control  much  more 
accurately  the  ripening,  and  to  reduce  the  number  of  failures.  The 
reason  why  the  inoculation  of  a  lactic  starter  tends  to  reduce  the 
failures  in  cheese-making  can  easily  be  understood  from  the  facts 
already  presented.  The  lactic  acid  bacteria  have  the  power  of 
checking  the  growth  of  other  germs,  and  even  of  destroying  them 
altogether.  When,  therefore,  the  milk  has  a  large  quantity  of 
lactic  bacteria  developing  rapidly  in  the  curd,  the  other  bacteria, 
which  might  under  different  circumstances  produce  putrefaction, 
are  prevented  from  increasing.  In  the  making  of  cheeses  this 
protecting  action  of  the  lactic  bacteria  becomes  very  important, 
and  is,  indeed,  the  secret  of  good  cheese.  The  cheese  remains  for 
weeks,  or  even  months,  in  a  moist  condition,  and  there  is  opportunity 
all  this  time  for  the  growth  of  bacteria.  If  a  proper  lactic  organism 
is  present  at  the  outset,  the  cheese  will  be  protected  from  the  various 
putrefactive  types  that  would  otherwise  surely  injure  it.  Their 
presence  in  sufficient  quantity  is  responsible  for  many  of  the  defects 
to  be  noticed  later. 

For  these  reasons,  then,  the  cheese  industry  is  learning  the 
prime  importance  of  a  strong  lactic  fermentation  in  milk  that  is  to 
be  converted  into  cheese,  and  in  order  to  bring  this  about  it  is 
rapidly  adopting  the  method  of  using  starters.  Cheese  starters  are 
essentially  identical  with  those  used  in  butter-making,  and  they  are 
used  in  much  the  same  way.     Home  starters  are  frequently  made 


THE    HARD    CHEESES.  20I 

and  inoculated  into  the  milk,  and  the  use  of  commercial  starters  is 
also  rapidly  growing.  It  is  interesting  to  find  that  the  types  of  lactic 
bacteria  that  are  useful  in  butter-making  are  not  always  satisfactory 
in  cheese-making.  Bacteria  that  give  a  fine  flavor  and  aroma  to 
butter  may  produce  a  bitter  taste  with  ruinous  results  when  used 
in  cheese-making.  The  use  of  starters  in  the  cheese  industry  seems 
to  be  firmly  established  at  the  present  time  and  is  practically  sure 
to  extend,  for  it  is  one  of  the  methods  of  safe-guarding  the  cheese 
against  undesirable  fermentations. 

"Faults"  in  Hard  Cheese. — The  value  of  a  cheese  is  wholly 
dependent  upon  the  success  of  the  ripening.  A  great  loss  is  entailed 
upon  cheese-makers  by  an  imperfect  ripening,  resulting  from  a 
variety  of  defects  called  faults.  These  are  commonly  due  to  the 
growth  of  certain  kinds  of  microorganisms  which  do  not  grow  in 
normal  cheeses.  The  injury  resulting  to  the  cheese  may  be  only 
sufficient  to  make  the  cheese  a  little  "off"  in  flavor  but  still  passable, 
or  it  may  be  so  great  as  utterly  to  ruin  the  cheese  and  make  it  a  total 
loss.  Some  of  these  faults  have  been  traced  to  their  sources  and 
will  be  considered  under  the  following  heads: 

Swelled  Cheese. — This   is,    perhaps,    the   most   common   fault. 
It  is  due  to  the  development  of  a  considerable  quantity  of  gas  which 
fills  the  curd  full  of  holes  and  causes  it  to  swell  and 
lose  its  shape.     Sometimes  the  holes  are  extremely      O  /?  O  \{ 
numerous  and  small,  and  sometimes  they  are  fewer  ^^  ^^^ 

but  of  larger  size.     In  any  case  they  are  undesir-        0 
able.     Even  good  cheeses  are  apt  to  show  some  gas 
holes,  but  so  few  as  to  do  no  special  injury.     Some-     bacillus    causing 
times   the   gas   is   so   abundant   as   to   cause  the     /^^J/^i^  cheese 

°  {B.  ShaTJert). 

cheese  to  burst,  in  which  case  it  is  completely 
ruined.  Between  these  extremes  are  all  kinds  of  intermediate 
grades.  The  development  of  the  gas  is  accompanied  by  an  unusual 
fermentation  and  an  unpleasant  taste  and  smell.  The  cause 
of  the  trouble  is  the  development  of  gas-producing  bacteria.  Several 
different  species  are  known  which  have  this  power  of  developing 
gas  in  great  quantities  in  the  ripening  cheese  (Fig.  43).  Most  of 
them,  perhaps  all,  belong  to  the  type  which  has  been  referred  to 


202        BACTERIA   AND    OTHER    MICROORGANISMS    IN    CHEESE. 

in  Chapter  XI  as  Bad.  aerogenes  type.  As  pointed  out  in  that 
chapter,  the  different  varieties  of  this  type  vary  much  in  the  amount 
of  gas  they  produce;  sometimes  the  quantity  is  very  slight,  sometimes 
it  is  extraordinarily  large;  and  it  is  easy  to  understand  how  different 
strains  and  different  quantities  of  bacteria  will  produce-grades 
of  gasiness  in  cheeses. 

Bitter  Cheese. — The  development  of  a  bitter  taste  is  one  of 
the  common  troubles  of  cheese-makers.  Sometimes  this  defect  will 
involve  the  whole  output  of  a  cheese  factory  and  cause  heavy  losses 
for  a  considerable  period.  Two  different  causes  have  been  deter- 
mined upon  as  responsible  for  the  trouble.  In  one  extended  series 
of  losses  thus  resulting,  the  cause  was  found  to  be  a  yeast  (Torula 
amari  Fig.  37)  that  obtained  access  to  the  cans  and  vats  and  con- 
tinued for  a  long  time  to  make  trouble.  The  difficulty  disappeared 
with  the  thorough  cleaning  and  sterilizing  of  all  articles  in  the  dairies 
and  factories.  In  another  series  of  bitter  cheeses  the  trouble  was 
found  to  be  in  one  of  the  liquefying  bacteria.  This  class  of  organisms 
is  nearly  always  present  in  milk,  and  though  the  growth  of  the  lactic 
bacteria  usually  crowds  them  out,  sometimes,  either  because  of 
their  extra  abundance  and  vigor  or  because  of  lack  of  sufficiently 
vigorous  acid  organisms,  they  are  not  overgrown  by  the  lactic  acid 
bacteria,  but  continue  to  multiply  until  they  develop  bitter  flavors 
that  injure  or  spoil  the  cheese.  This  cause  of  bitterness  has  been 
detected  in  both  the  hard  and  the  soft  cheeses. 

Putrid  Cheese. — Sometimes  soft  spots  appear  upon  the  surface 
of  a  cheese.  They  may  become  larger,  eating  their  way  into  the 
cheese,  and  producing  a  more  or  less  slimy  appearance.  The 
trouble  is  undoubtedly  due  to  the  growth  of  putrefying  bacteria, 
but  not  much  is  known  about  the  matter  at  present. 

Fruity  or  Sweet  Cheese. — This  is  a  phenomenon  which  occurs 
sometimes  over  widely  extended  .districts  and  detracts  from  the 
character  of  the  cheese  without  always  ruining  it.  It  is  character- 
ized by  a  peculiar  sweet  taste,  which,  although  not  unpleasant, 
spoils  the  flavor  of  the  cheese  and  thus  injures  the  sale  of  the  prod- 
uct. This  trouble  has  been  found  to  be  due  to  a  yeast  which  gets 
into  the  milk. 


SOFT    CHEESES.  203 

Rusty  spot. — This  is  characterized  by  rusty,  red  spots  on  the 
outside  and,  indeed,  not  infrequently  throughout  the  whole  cheese. 
The  cheese  loses  its  value  and  may  in  the  end  become  quite  ruined 
if  the  trouble  develops  sufficiently.  The  cause  is  a  bacterium,  B. 
rudensis. 

Many  other  "faults"  may  be  recognized  as  interfering  with  the 
normal  ripening.  Black  spots  and  blue  spots  are  sometimes  noticed 
and  a  variety  of  "off"  flavors  that  cannot  be  described.  In  regard 
to  all  of  these  troubles  the  cheese-maker  has  the  serious  disad- 
vantage that  they  cannot  be  discovered  until  the  ripening  has  be- 
come partially  completed,  and  then  it  is  too  late  to  apply  any 
remedy.  The  method  of  meeting  them  must  be  by  prevention 
rather  than  cure^  and  after  an  improper  ripening  has  begun,  practi- 
cally nothing  can  be  done  to  stop  it.  By  cleanliness,  by  frequent 
sterilization  of  vats,  and  by  the  use  of  vigorous  lactic  acid  starters 
much  can  be  done  to  prevent  these  troubles,  but  there  seems  to  be 
no  remedy  after  the  improper  ripening  has  begun.  A  vigorous 
lactic  acid  starter  will  go  far  toward  preventing  gassy  cheese,  and 
the  slimy  whey  used  in  the  Edam  cheese  prevents  it  in  that  particular 
brand.  A  regulation  of  temperature  during  ripening  will  also  aid, 
since  the  gassy  organisms  grow  best  at  higher  temperatures,  while  at 
a  lower,  about  60°,  the  common  lactic  acid  bacteria  are  the  more 
vigorous.  When  to  these  two  suggestions  of  vigorous  starters  and 
cool  temperatures  we  add  great  care  in  keeping  all  milk-vessels  clean, 
and  in  sterilizing  frequently  by  steam,  we  have. included  practically 
the  only  methods  in  use  of  much  significance  in  guarding  against 
the  various  cheese  faults. 

SOFT  CHEESES. 

The  essential  difference  between  a  hard  cheese  and  a  soft  cheese 
is  that  the  latter  has  a  very  much  higher  percentage  of  moisture. 
To  bring  about  this  condition,  the  method  of  manufacture  is  de- 
signed to  retain  the  whey  in  the  curd.  After  the  milk  is  curdled  the 
curd  is  sometimes  dipped  out  directly  into  forms  provided  with 
holes  in  their  sides,   through  which   the  whey    drains    naturally 


204        BACTERIA  AND    OTHER    MICROORGANISMS    IN    CHEESE. 

without  the  application  of  any  pressure.  In  other  cases  the  curd  is 
cut  in  the  vat,  but  the  curd  and  whey  together  are  dipped  into  forms 
for  draining.  As  a  resuh,  there  is  produced  a  cheese  which  con- 
tains a  much  greater  amount  of  water  than  is  allowed  to  remain  in 
the  hard  cheeses. 

This  large  amount  of  water  produces  a  ripening  of  a  totally 
dififerent  character  from  that  which  occurs  in  the  hard  cheeses.  The 
process  of  ripening  is  different,  the  agents  that  bring  it  about  are 
different,  and  the  final  results  are  very  different  from  those  in  the 
hard  cheeses.  Moreover,  the  ripening  is  liable  to  greater  variations 
in  the  soft  cheeses  than  in  the  hard  cheeses,  and  it  is  more  difficult  to 
control,  because  of  the  presence  of  so  much  moisture.  Bacteria, 
yeasts,  and  molds  all  find  a  suitable  medium  for  growth  in  the  wet 
curd  of  the  soft  green  cheese,  and  unless  the  progress  of  the  ripening 
is  exactly  right,  the  cheese-maker  may  expect  the  development  of 
kinds  of  microorganisms  that  are  unfavorable  to  his  product  and 
that  will  spoil  his  cheeses.  Soft  cheeses  are  much  less  uniform  in 
character  than  hard  cheeses.  They  differ  very  greatly  in  texture 
and  flavor,  and  are  subject  to  various  defects  that  injure  or  ruin 
them.  They  are,  in  short,  more  difficult  to  make  with  success  than 
the  hard  cheeses,  largely,  if  not  wholly,  because  the  water  they  con- 
tain offers  such  a  favorable  medium  for  the  growth  of  bacteria  and 
other  microorganisms. 

The  ripening  of  several  of  these  cheeses  has  been  carefully 
studied  by  bacteriologists.  A  brief  description  of  the  phenomena 
in  three  of  these  will  illustrate  the  principles  concerned  The  three 
described  represent  three  types  of  soft  cheeses. 

Camemhert  Cheese. — This,  together  with  Brie  cheese  and  some 
others,  represents  a  type  in  which  the  ripening  is  due  partly  to 
bacteria  in  the  curd  and  partly  to  molds  growing  on  the  surface  of 
the  cheese,  but  not  penetrating  below  the  surface.  The  cheeses  are 
small,  about  four  inches  in  diameter  and  one  inch  thick.  The 
soft  curd  is  dipped  out  with  the  whey  into  forms  and  allowed  to 
drain  by  its  own  weight.  The  cheese,  with  its  large  moisture  con- 
tent, is  then  allowed  to  ripen  in  a  damp  room  where  the  temperature 
is  low.     Sometimes  two  rooms  of  different  temperatures  are  used. 


SOFT    CHEESES. 


205 


The  ripening  occupies  from  five  to  eight  weeks,  and  a  series  of 
changes  takes  place  in  the  cheese,  of  which  the  following  is  a 
summary: 

The  first  phenomenon  that  occurs  is  the  souring  of  the  curd, 
which  is  brought  about  by  the  Bad.  lactic  acid  type  of  organism. 
These  bacteria  grow  in  the  milk  previous  to  the  addition  of  the 
rennet,  although  the  milk  is  not  allowed  to  become  very  sour  before 
curdling.  But  they  continue  to  grow  during  the  curdling  and  for  a 
day  or  two  after  the  cheese  is  made.  If  by 
any  chance  the  lactic  bacteria  fail  to  develop 
vigorously,  a  ruined  cheese  is  sure  to  result. 

The  second  step"  in  the  ripening  is  the  ap- 
pearance on  the  surface  of  the  cheese  of  a 
species  of  mold,  which  has  been  named 
Penicillium  camemberti  (Fig.  44).  This  mold 
appears  in  from  two  to  four  days  and  is  at  first 
of  a  pure  white  color;  later,  when  it  begins  to 
produce  spores,  it  becomes  a  steel  gray,  but 
never  a  deep  blue  like  the  common  mold.  It 
is  a  species  of  mold  that  apparently  does  not 
occur  in  America,  but  is  very  common  in 
Europe  in  those  sections  where  Camembert 
cheese  is  made.  Its  absence  from  America  is 
the  chief  reason  why  this  country  has  been  unable  to  make 
Camembert  cheese.  Where  successful  cheese  of  this  type  has  been 
produced,  it  has  been  by  importing  and  inoculating  this  type  of 
mold  into  the  American  cheese  factories.  This  white  mold  grows  on 
the  surface  of  the  cheese,  but  does  not  penetrate  below  the  surface. 
After  about  two  weeks  it  reaches  its  limit  of  growth,  forms  spores, 
and  dries  down  to  a  somewhat  thin  crust.  The  growth  of  this  mold, 
together  with  other  organisms,  neutralizes  the  acid  of  the  curd  at 
the  surface  of  the  cheese  and  renders  it  slightly  alkaline. 

In  the  meantime  the  mold  has  secreted  an  enzyme  which  has 
the  power  of  digesting  the  curd.  As  fast  as  the  acidity  of  the  curd 
is  reduced  and  the  enzyme  secreted,  the  latter  acts  upon  the  curd, 
changing  it  from  a  hard  consistency  to  a  soft  texture.     At  first  the 


Fig.  44. — Penicillium 
camemhertii,  the  mold 
ripening  camembert 
cheese  {Thorn). 


2o6        BACTERIA  AND    OTHER    MICROORGANISMS    IN    CHEESE. 


cheese  is  a  hard,  solid  curd  from  surface  to  center;  but  as  this  enzyme 
acts  beneath  the  mold  there  is  formed  a  thin  layer  of  soft  material. 
This  layer  grows  deeper  and  deeper  as  it  encroaches  upon  the  curd. 
The  enzyme  produces  a  profound  change  in  the  casein,  converting 
it  first  into  peptones  and  similar  bodies;  later  these  break  down 
into  simpler  bodies,  or  end-products,  among  which  ammonia  may 
always  be  detected.  These  latter  end-products  give  the  flavor, 
and  appear  to  be  produced  by  bacteria  rather  than  by  the  action  of 
the  enzymes  secreted  by  the  mold.  During  the  ripening  the  cheese 
will  be  found  to  have  a  core  of  a  sour,  acid  curd  in  the  center, 
surrounded  by  a  layer  of  soft,  digested  material.  The  cheese  ripens 
thus,  from  the  surface  inward,  and  is  not  completely  ripened  until 
the  soft  layer  reaches  the  center. 

The  flavors  are  not  due  to  the  enzyme  diges- 
tion,'but  to  the  end-products  of  decomposition. 
In  the  case  of  this  cheese,  as  in  the  hard  cheeses, 
no  positive  knowledge  is  at  hand  as  .to  the  exact 
source  of  the  flavor.  That  it  is  not  due  to  the 
mold  alone  is  certain,  from  the  fact  that  the 
softened  cheese  may  be  nearly  tasteless,  if  a  pure 
culture  of  mold  has  completed  the  ripening.  The 
peculiar  Camembert  flavor  is,  beyond  doubt,  as- 
sociated with  some  of  the  microorganisms  grow- 
ing in  or  on  the  cheese,  but  at  present  no  more 
is  known  about  the  matter. 

Roquefort  Cheese. — This  represents  a  type  of 
cheese  that,  like  Camembert,  is  ripened  by  both 
bacteria  and  molds.  Closely  allied  to  it  are  the 
Stilton  and  Gorgonzola  cheeses.  The  mold  is  a 
blue  instead  of  a  white  one,  and  it  grows  through  the  cheese  and  not 
alone  on  its  surface.  To  bring  about  the  growth  in  the  center  of 
the  cheese  special  means  are  devised  in  its  manufacture.  The 
cheese-maker  begins  by  cultivating  the  necessary  mold  on  bread. 
After  the  mold  on  the  bread  has  produced  a  great  quantity  of  spores, 
the  mass  is  dried  and  ground  into  powder  (Fig.  45).  After  curdling 
the  milk  with  rennet  in  the  usual  way  and  draining  the  curd,  it 


Fig.  45. — PenicU- 
lium  roquefortii,  the 
mold  ripening  Roque- 
fort cheese  {Thorn). 


SOFT    CHEESES.  207 

is  placed  in  a  form  in  a  thin  layer,  and  over  the  top  of  the  layer  is 
strewn  a  quantity  of  powdered,  moldy  bread  with  its  thousands  of 
spores.  Over  this  is  placed  another  layer  of  curd  with  more 
mold  spores;  then  a  third  layer  of  curd  over  all.  The  mold  is  thus 
planted  within  the  cheese.  The  whole  is  then  pressed  bymoderate 
pressure  in  a  form.  After  a  few  days  the  cheese  becomes  hard 
enough  to  be  removed  from  the  form  and  is  next  placed  upon  a 
machine  which  punches  it  full  of  holes  by  means  of  small  needles. 
The  purpose  of  this  is  to  allow  air  to  enter  into  the  center  of  the 
cheese,  thus  furnishing  the  molds  in  the  center  with  the  air  they  need 
for  growth.  The  cheese  is  then  put  into  the  ripening  room,  where 
the  molds  develop,  growing  primarily  within  the  cheese.  As 
the  molds  grow  they  develop  a  peculiarly  peppery,  piquant  taste, 
which  is  characteristic  of  the  Roquefort  cheese.  Just  before  the 
cheese  is  fully  ripe  it  tastes  bitter;  but  this  taste  disappears  as  the 
final  flavor  develops.  A  good  Roquefort  cheese  is  only  possible 
when  there  is  a  luxuriant  growth  of  these  molds  within  the  cheese, 
no  surface  growth  being  allowed. 

The  successful  manufacture  of  the  Roquefort  cheese  in  the 
United  States  is  yet  to  come.  There  seems  to  be  no  reason  why 
a  cheese  cannot  be  made  in  this  country  which,  if  ripened  by  the 
Roquefort  mold,  will  have  the  Roquefort  flavor;  but  it  is  not  likely 
that  a  real  Roquefoit  can  ever  be  made  in  America  because,  the 
typical  Roquefort  is  made  of  sheep's  milk,  and  it  is  doubtful  if 
Americans  will  ever  be  content  to  raise  sheep  and  milk  them.  Stilton 
and  Gorgonzola  cheeses  are,  however,  made  from  cow's  milk  and 
ripened  by  the  same  mold  that  is  found  in  the  Roquefort.  These 
cheeses  can  certainly  be  made  in  this  country.  Stilton  has  already 
been  made  in  Canada,  and  there  is  no  reason  why  its  manufacture 
cannot  be  undertaken  and  developed  in  the  United  States. 

The  Limhurger  Cheese. — This  represents  a  type  of  cheese  in 
which  molds  play  no  part  in  the  ripening,  but  bacteria  are  the 
primary  and  perhaps  the  sole  agents. 

After  being  drained  in  a  mold,  until  firm  enough  to  be  handled 
the  cheeses  are  placed  in  a  ripening  cellar.  Every  few  days  they 
are  removed  from  the  shelves  and  rubbed  over  with  some  liquid. 


2o8        BACTERIA   AND    OTHER    MICROORGANISMS    IN    CHEESE. 

water  being  commonly  used,  although  vinegar  is  sometimes  put  into 
the  water.  The  surface  is  thus  kept  constantly  moist.  Because 
of  this  constant  moisture  on  the  surface  of  the  cheese,  molds  cannot 
grow  upon  it,  for  they  need  a  damp,  not  a  wet  surface;  but  a  quantity 
of  bacteria  grow  instead.  These  ripen  the  cheese,  doubtless  by 
the  secretion  of  chemical  ferments,  although  the  process  has  not 
as  yet  been  fully  studied.  The  resulting  cheese  develops  very 
high  flavors,  closely  resembling  those  of  decay,  and  the  cheeses 
rapidly  putrefy  when  they  become  old.  If  they  are  marketed 
at  the  right  stage  the  flavors  are  not  strong  enough  to  be  disagreeable, 
and  many  persons  are  very  fond  of  them.  The  Limburger  type 
of  cheese  includes  Backstein  and  some  others. 

PRACTICAL  RESULTS. 

The  practical  application  of  bacteriology  to  cheese-making  is 
just  in  its  infancy,  and  it  is  quite  impossible  to  determine  the  extent 
of  its  development  in  the  future.  As  already  pointed  out,  cheese- 
makers,  in  the  last  few  years,  have  been  using  pure  cultures  of  lactic 
acid  bacteria  as  starters  to  insure  a  more  complete  and  more  uniform 
souring  of  the  curd.  This  practice  is  rapidly  growing.  The  lactic 
starters  have  two  purposes.  First,  the  growth  of  the  acid  organisms 
checks  the  growth  of  other  bacteria  that  would  be  likely  to  spoil 
the  cheese,  and  this  check  seems  to  be  quite  necessary  for  the  proper 
ripening  and  for  the  preventing  of  faults.  Second,  the  formation 
of  lactic  acid  appears  to  be  a  needed  step  in  the  chemical  changes 
that  constitute  the  ripening.  Hence,  the  use  of  a  good  starter  of 
acid  organisms  has  a  reasonable  foundation,  and  we  may  confidently 
assume  that  the  practice  of  using  starters  will  increase.  In  the 
making  of  the  Edam  cheese  of  Holland,  the  practice  of  using  slimy 
whey  to  aid  the  ripening  of  the  cheese  has  become  very  extended. 
The  slimy  whey  is  a  culture  of  bacteria  in  whey,  and  is,  therefore, 
simply  another  method  of  using  a  bacterial  starter  to  control  the 
cheese-ripening.  It  hastens  the  ripening  and  makes  it  more  uniform, 
but  it  does  not  improve  the  cheese.  In  the  manufacture  of  Roque- 
fort cheese,  mold  cultures  are  intentionally  added  to  bring  about 


PRACTICAL    RESULTS.  209 

the  proper  ripening,  and  in  recent  years  pure  cultures  of  molds  have 
been  applied  in  the  manufacture  of  cheeses  of  the  Camembert 
type.  In  these  cheeses,  too,  lactic  acid  starters  are  very  commonly 
used.  From  these  few  instances  it  is  evident  that  the  practical 
application  of  bacteriology  to  cheese-making  has  already  begun, 
and  it  is  almost  certain  that  a  large  field  is  open  for  the  future  in  this 
direction. 


18 


PART  IV. 

RELATION  OF  MICROORGANISMS   TO  MISCELLA- 
NEOUS FARM  PRODUCTS. 


CHAPTER  XV. 


ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE, 

FLAX. 


Although  the  problems  of.  soil  fertility  and  dairying  offer  the 
largest  field  for  the  application  of  bacteriology  to  farm  life,  there  are 
many  other  problems  of  minor  importance  where  microorganisms 
play  a  part.  Most  of  these  concern  food  products,  either  their 
preparation  or  preservation,  although  some  have  no  relation  to 
foods. 

In  all  temperate  and  cold  climates  it  is  necessary  to  preserve  food 
for  the  winter  season.  This  applies  equally  to  the  farmer's  own 
food  and  to  that  of  his  cattle. 
There  is  a  difficulty  in  preserving 
some  kinds  of  food  because  of  the 
readiness  with  which  putrefactive 
bacteria  cause  their  disintegration 
and  decay.  Bacteria  will  feed  upon 
almost  any  kind  of  organic  matter, 
provided  there  is  plenty  of  moisture 
at  hand;  but  some  of  the  foods,  like 
most  grains,  have  such  a  small 
amount  of  water  in  them  that  bacteria  are  unable  to  grow,  and 
thei;e  .  is  little  difficulty  in  preserving  these  for  an  indefinite 
length  of  time.  Nature  herself,  at  the  end  of  the  growing  season, 
extracts  the  water  from  the  seeds,  leaving  the  comparatively  dry 

211 


Fig.  46. — Showing  nature's  method 
of  preserving  seeds  by  drying.  The 
lower  figures  are  dried  and  the  upper 
are  fresh  seeds. 


212     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

mass  to  remain  over  the  period  of  rest  until  the  growing  time  comes 
again  (Fig.  47).  Other  kinds  of  food  contain  a  large  amount  of 
water,  and  the  farmer  must  find  some  means  of  protecting  such  food 
from  bacterial  action.  This  is  accomplished  in  a  variety  of  ways, 
but  may  best  be  considered  under  two  heads:  i.  The  agency  of 
microorganisms  in  preparing  the  crop.  2.  The  methods  of  pro- 
tecting the  crop  from  the  attack  of  mischievous  organisms.  To  a 
certain  extent  the  two  subjects  overlap,  since  in  several  cases  the 
methods  adopted  for  preserving  the  material  furnish  it  with  flavors 
or  other  characters  which  distinctly  add  to  its  value. 

THE  ALCOHOLIC  FERMENTATION. 

The  fermentation  of  sugar  into  alcohol  and  carbonic  acid  is  due 
to  yeast.  Among  the  various  aspects  of  farm  life  there  are  quite  a 
number  based  upon  this  type  of  fermentation.  We  have  seen  that 
yeasts  are  especially  related  to  sugars,  and  that  any  product  which 
contains  much  sugar  is  more  likely  to  undergo  alcoholic  fermenta- 
tion than  putrefaction.  Yeasts,  when  dry,  may  remain  alive  for  a 
long  time  and  float  around  in  the  air.  The  air  at  all  times  and  in 
almost  all  places  is,  therefore,  sure  to  contain  these  living  yeast 
plants,  ready  to  begin  to  grow  and  produce  a  fermentation  whenever 
they  fall  into  a  sugar  solution.  These  air  yeasts  are  sometimes 
called  wild  yeasts  in  distinction  from  the  cultivated  yeasts  that  are 
now  articles  of  commerce.  But  whether  from  the  air  or  from  a 
package  of  commercial  yeast,  the  organisms  are  essentially  the  same 
and  their  action  the  same. 

The  chief  products  of  our  farms  which  are  liable  to  direct  alcoholic 
fermentation  are  the  fruit  juices.  Alcoholic  fermentation  is  also 
the  foundation  of  the  gigantic  distillery  and  brewery  industries,  which 
make  use  of  grains  and  other  farm  products.  But  these  hardly 
belong  to  our  immediate  subject.  There  are,  however,  a  few  forms 
of  fruit-juice  fermentations  more  or  less  common  on  our  farms. 

Wines. — The  name  wine  is  given  to  the  fermented  juice  of 
fruits.  The  most  common  fruit  used  for  this  is  the  grape,  whose 
juice  is  rich  in  sugar  and  easily  pressed  from  a  mass  of  the  fruit,  as  a 


THE   ALCOHOLIC    FERMENTATION.  213 

clear  liquid.  Upon  the  skin  of  the  grape  there  are  sure  to  collect, 
during  its  growth,  a  variety  of  microorganisms,  mostly  from  the  air, 
and  among  them  will  be  enough  yeasts  to  start  a  fermentation  of  the 
sugars  as  soon  as  the  juice  is  extracted  from  the  grape.  The  grape 
juice,  therefore,  needs  no  yeast  added  to  it  to  start  a  fermentation, 
since  the  wild  yeasts  are  sufficient  to  give  all  the  inoculation  neces- 
sary. In  the  making  of  wines  the  usual  method  is  simply  to  press 
out  the  juice  from  the  grape  and  then  to  allow  a  spontaneous  fermen- 
tation to  occur.  Occasionally  the  practice  of  adding  yeast  to  the 
juices,  in  order  to  hasten  or  control  the  fermentation,  has  been 
recommended.  This  method,  which  has  made  a  complete  revolu- 
tion in  the  brewery  industries,  has  not,  as  yet,  been  very  extensively 
applied  to  wine-making.  The  knowledge  that  there  are  many 
kinds  of  yeasts  with  different  values  in  fermenting,  certainly  suggests 
that,  in  wine-making,  an  improvement  may  be  anticipated  by  this 
use  of  pure  cultures.  The  use  of  pure  cultures  in  wine-making  is 
becoming  more  common,  and  where  they  have  been  used  an  im- 
proved product  or  a  better  control  has  been  claimed. 

Some  farms,  where  grapes  are  raised  in  abundance,  prepare  for 
market  an  unfermented  grape  juice  which  is  essentially  wine  that 
has  not  been  allowed  to  ferment.  The  expressed  juice  is  sterilized 
by  a  temperature  of  about  170°,  which  is  sufficient  to  destroy  the 
yeast  cells  and  to  prevent  fermentation,  if  the  juice  be  subsequently 
kept  from  further  contamination  by  being  bottled.  The  principle 
concerned  is  simple,  but  there  are  various  practical  difl&culties  in  the 
way  that  make  it  difficult  to  produce  a  good  quality  of  grape 
juice. 

The  term  wine  usually  refers  to  the  fermented  juice  of  the 
grape.  But  in  sections  of  the  country  where  grapes  are  not  exten- 
sively grown  other  fruit  juices  are  used.  Wines  are  made  from  the 
juice  of  blackberries,  currants,  raspberries, ^elder  berries,  etc.  In  the 
making  of  wine  from  these  fruits,  since  the  juice  is  not  very  sweet, 
sugar  is  commonly  added  in  amounts  varying  with  the  sweetness  of 
the  fruit  and  depending  also  on  whether  a  sweet  or  sour  wine  is 
desired.  The  mixture  is  then  generally  left  to  ferment  spontaneously 
under  the  influence  of  the  wild  yeasts  that  are  abundant  enough  to 


214     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

produce  a  vigorous  action.  Yeast  is  sometimes  added.  As  a  rule, 
the  fermentation  is  allowed  to  continue  as  long  as  it  will,  after  which 
the  wine  is  bottled  and  thus  preserved  for  use. 

Cider. — This  is  nothing  but  apple  wine,  and  is  made  in  large 
quantities  in  sections  of  the  country  where  apples  are  abundant. 
The  expressed  apple  juice  is  seldom  treated  at  all,  but  left  to  ferment 
spontaneously.  The  amount  of  sugar  in  apple  juice  is  small,  and 
the  completely  fermented  product  contains  a  proportionately  small 
amount  of  alcohol.  Sweet  cider  is  a  name  given  to  the  product 
while  it  is  still  fermenting;  it  contains  but  a  small  amount  of  alcohol, 
but  is  filled  with  the  carbon  dioxid  gas  that  is  produced  by  the 
fermentation.  Hard  cider  is  the  name  applied  after  the  fermenta- 
tion is  nearly  or  quite  over,  when  the  evolution  of  CO2  has  ceased 
and  the  alcohol  is  at  its  maximum.  In  the  making  of  cider,  as  in 
most  other  fermentations,  great  improvements  have  been  made  in 
recent  years  by  the  application  of  the  discoveries  of  bacteriologists. 
The  use  of  pure  cultures  of  yeasts,  in  the  place  of  spontaneous 
fermentation,  makes  the  product  of  a  better  character  and  the  fer- 
mentation more  uniform.  Numerous  other  improvements  have 
been  made  in  the  details,  so  that  this  product,  formerly  made  on  the 
farm  in  a  haphazard  fashion,  without  care  and  with  little  or  no 
knowledge  of  the  processes,  is  now  made  on  a  larger  scale  in  special 
cider  factories,  resulting  in  a  cider  of  a  much  higher  quality. 

Yeasts  in  Bread  Raising. — The  most  common  use  of  yeast 
is  in  the  raising  of  bread.  All  nations  and  all  peoples  have  been 
accustomed  to  make  bread  from  the  flour  of  different  grains.  The 
earliest  method  was  simply  to  stir  the  flour  in  water  and  bake 
the  mixture  into  a  hard,  unleavened  bread.  The  next  step  was 
to  allow  the  dough  to  stand  for  a  number  of  hours  in  a  warm  place 
until  it  became  somewhat  swollen  by  the  gas  formed  within  it, 
and  then. to  bake  it.  The  gas  made  the  dough  porous  and  resulted 
in  a  bread  filled  with  holes,  easier  to  masticate,  of  better  flavor,  and 
more  easily  digested.  This  was  leavened  bread.  The  next  step 
was  to  take  a  little  of  the  leavened  dough  and  mix  it  with  the  next 
lot  of  fresh  dough,  to  hastenen  the  leavening,  a  process  that  was 
simply  inoculating   the   dough   with   the  yeast  organisms   in   the 


THE   ALCOHOLIC    FERMENTATION.  21$ 

leaven.  The  next  step,  taken  a  long  time  afterward,  was  to  discover 
that  it  is  the  yeast  in  the  leaven  which  produces  the  raising  of 
the  bread,  and  then  to  separate  the  yeast  from  other  undesired 
materials  in  the  leaven,  and  use  it  in  pure  cultures.  This  finally 
gave  the  yeast  that  has  been  used  for  half  a  century  or  more.  The 
use  of  leaven  has  not  altogether  disappeared,  but  yeast  is  quicker 
and  more  reliable. 

The  action  of  the  yeast  in  bread-raising  is  very  simple.  The 
dough  contains  a  considerable  quantity  of  starch  and  also  a  small 
quantity  of  diastase,  an  enzyme  capable  of  converting  starch  into 
sugar.  The  yeast  acts  upon  the  sugar  thus  produced  and  forms 
from  it  alcohol  and  carbon  dioxid.  The  latter,  being  a  gas, 
forms  bubbles  in  the  dough,  causing  it  to  swell  and  become  lighter. 
When  subsequently  baked,  these  gas  bubbles  leave  their  traces  in 
the  numerous  holes  that  one  finds  in  raised  bread. 

In  the  raising  of  bread  the  practice  of  depending  on  the  wild 
yeasts  of  the  air  has  long  since  disappeared  and  yeast  cultures  are 
now  almost  universally  used.  These  commercial  yeasts  have  been 
chosen  from  the  considerable  variety  of  yeasts  known,  and  are, 
of  course,  the  ones  that  have  been  found  to  produce  the  best  results 
in  bread-raising.  They  are  not  the  same  varieties  as  those  found 
best  for  brewing.  The  yeast  is  cultivated  in  large  quantity  and 
then  put  up  in  a  convenient  form  for  distribution,  sometimes  dried, 
in  which  condition  it  will  keep  alive  for  weeks,  and  sometimes 
compressed  into  a  moist  cake,  compressed  yeast,  in  which  condition 
it  will  keep  only  a  few  days. 

Bacterial  Impurities  in  Bread-raising. — The  yeast  cakes  are 
never  pure  yeast,  but  may  contain  undesired  bacteria,  which  then 
get  into  the  dough  and  sometimes  produce  trouble.  Occasion- 
ally, too,  such  bacteria  may  get  into  the  dough  from  other  sources 
than  yeast,  such  as  dirty  water  or  dirty  utensils  in  the  kitchen. 
During  the  raising,  lactic  acid  bacteria  always  grow,  and  they 
seem  to  be  necessary  in  order  to  prevent  the  growth  of  other  species 
of  more  troublesome  organisms.  Sometimes  such  bacteria  grow 
too  vigorously  and  may  cause  trouble.  At  least  two  different 
faults  in  bread-making  are  known  to  be  caused  by  undue  growth 


2X6     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

of  bacteria.  The  first  of  these  is  sour  bread,  due  to  the  growth  in 
the  dough  of  acid-forming  bacteria.  This  occurs  most  commonly 
when  the  raising  is  allowed  to  continue  too  long  or  at  too  high  a 
temperature.  The  bacteria  in  question  will  not  produce  trouble 
if  fresh  yeast  be  used  and  the  raising  be  completed  in  less  than 
eight  hours'  time.  A  second  fault,  due  to  bacteria,  is  slimy  bread. 
This  is  of  rare  occurrence,  though  sometimes  it  will  infect  a  bakery 
and  continue  day  after  day.     This  bread,  when  fresh,  appears  normal ; 

ybut  after  a  few  hours  it  becomes  slimy,  so  that, 
when  broken,  it  appears  as  if  filled  with  cobwebs. 
^\     The  trouble  is  due  to  bacteria,  the  source  of  which 
t         may  be  either  the  utensils,  the  flour,  or  the  yeast 
Fig.  47— The    (Fig-  47).     The  remedy  is  in  cleaning  and  steriliz- 
bacteria  that  cause    j^g  all  baking  dishes.     If  this  does  not  remove  the 

shmy  bread.  ,  ,      .     •  1,  ,  ,       ,  1      <•  n 

trouble  it  is  well  to  change  the  brand  of  flour  or 
get  a  fresh  supply  of  yeast.  The  trouble  may  also  be  relieved  by 
keeping  the  bread  cold  so  as  to  prevent  bacteria  growth.  The 
addition  of  a  little  lactic  acid  to  the  dough,  or  the  use  of  equal 
parts  of  sour  whey  and  water  in  mixing  the  dough,  is  also  recom- 
mended as  a  remedy,  based  upon  the  fact  that  lactic  acid  bacteria 
will  check  the  growth  of  other  organisms. 

VINEGAR-MAKING. 

Vinegar  is  used  both  as  a  direct  condiment  to  give  relish  to  foods 
and  as  a  preservative,  as  in  the  manufacture  of  pickles.  It  is  made 
on  a  large  scale  in  vinegar  factories  and  on  a  small  scale  on  farms. 
It  is  always  made  from  some  weak  alcoholic  solution,  like  cider, 
weak  wine,  or  beer,  each  locality  using  as  a  source  the  alcoholic 
solution  most  easily  obtained.  The  essential  part  of  the  process 
is  the  chemical  union  of  the  alcohol  with  oxygen  from  the  air,  by 
which  it  is  converted  into  acetic  acid.  Such  a  simple  oxidation  can 
be  brought  about  by  a  purely  chemical  process.  As  long  ago  as 
1 72 1,  Davy  discovered  that  platinum-black,  or  finely  divided  plati- 
num, when  mixed  with  alcohol,  causes  an  active  union  with  oxygen 
to  take  place,  which  results  in  the  production  of  acetic  acid. 


VINEGAR-MAKING.  21 7 

When  vinegar  is  formed  in  the  usual  way,  a  brownish  gelatinous 
mass,  called  "mother  of  vinegar"  is  formed,  and  for  a  time  it  was 
believed  that  this  mother  of  vinegar  acted  like  the  platinum-black, 
"condensing"  the  oxygen,  and  thus  causing  a  chemical  union. 
But  the  error  of  this  conclusion  was  later  shown,  i.  The  ordinary 
vinegar  fermentation  is  stopped  by  the  accumulation  of  acid,  and 
it  will  not  occur  at  all  if  the  amount  of  acetic  acid  in  the  solution  be 
more  than  14  per  cent.  The  formation  of  acid  by  platinum-black  is 
entirely  uninfluenced  by  such  an  accumulation.  2.  The  formation 
of  the  acid  in  vinegar  is  most  abundant  at  about  95°  F.,  diminishing 
rapidly  at  higher  temperatures,  and  may  in  itself  produce  so  much 
heat  as  actually  to  ignite  the  alcohol.  3.  Later  the  production 
of  acetic  acid  by  the  growth  of  pure  cultures  of  certain  bacteria 
showed  vinegar-making  to  be  a  true  fermentation. 

The  Vinegar  Organism. — The  mother  of  vinegar  is  a  soft, 
semi-solid  mass,  commonly  forming  a  scum  on  the  surface  of  the 
fermenting  alcohol.  Vinegar  is  not  forrned  if  this  material  be  lack- 
ing, and  a  very  small  bit  placed  on  the  surface  of  an  alcoholic  solution 
soon  extends  itself  and  covers  the  whole  surface,  inducing  an  active 
acetic  acid  formation.  This  mother  of  vinegar  proves  to  be  a  mass 
of  microorganisms.  It  was  first  named  Mycoderma  by  Persoon, 
who  studied  it  in  181 2,  without  having  any  suspicion  that  the  skin 
was  the  cause  of  the  acetic  acid.  Later,  Kiitzing,  showed  that  this 
skin  was  made  of  numerous  minute,  living  organisms,  and  positively 
asserted  that  they  were  the  cause  of  the  acetic  fermentation. 
But  the  chemist  Liebig  checked  the  advance  of  discovery  by  his  own 
theories  of  fermentation,  which  regarded  the  whole  class  of  phe- 
nomena as  chemical  processes. 

It  was  eventually  Pasteur  who  demonstrated  that  the  process  is 
really  a  fermentation  due  to  the  activity  of  the  microorganisms  in  the 
mother  of  vinegar.  An  examination  of  this  material  shows  it  to  be 
made  of  a  mass  of  bacteria.  Pasteur  used  for  them  the  name 
Mycoderma  aceti,  but  he  did  not  study  them  sufficiently  to  show 
what  they  were,  although  he  demonstrated  their  relations  to  vinegar- 
making.  Hansen  later  proved  they  were  bacteria,  and  showed 
that  in  the  different  samples  of  "mother"  there  are  several  varieties. 
19 


2l8     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

Henneberg  still  later  increased  our  knowledge  of  the  bacteria,  so 
that  we  now  know  that  there  are  at  least  fifteen  varieties  of  these 
vinegar  organisms,  differing  from  each  other  in  various  respects, 
such  as  their  optimum  temperatures,  the  amount  of  acid  they  will 
produce,  etc.  Bact.  aceti,  Bad.  pasteurianum,  Bad.  kiltzingianum, 
Bad.  xylinoides,  Bad.  orleanense,  and  Bad.  xylinum  are  names 
applied  to  the  most  important  types. 

All  of  these  acetic  bacteria  have  certain  very  characteristic 
points  which  they  share  in  common,  thus  forming  a  group  by  them- 
selves.    They  all  exist  in  three  different  forms,  shown  in  Fig.  48. 


Fig.  48. — Acetic  acid  bacteria,  showing  long  rods  and  rounded  swollen  centers. 

They  may  form  chains  of  short  rods,  looking  like  ordinary  bacilli. 
At  high  temperatures  they  grow  out  in  long  slender  threads,  some- 
times very  long,  without  any  traces  of  divisions.  These  threads 
may  subsequently  break  up  into  short  elements.  At  low  tempera- 
tures they  have  a  peculiar  habit  of  forming  long  threads  with  rounded 
swollen  centers.  When  these  threads  break  up  into  short  forms 
only  the  thick,  swollen  centers  remain  undivided.  This  character 
is  such  a  peculiar  one  that  it  places  these  acetic  bacteria  in  a  class  by 
themselves.  These  different  varieties  are  distinguished  not  only  by 
slight  differences  in  structure,  but  also  by  some  important  dif- 
ferences in  relation  to  condition,  and  in  their  power  of  forming 
acetic  acid.  They  vary  in  the  amount  of  acetic  acid  they  will  pro- 
duce under  similar  conditions.     For  example,  B.  aceti  produces 


VINEGAR-MAKING.  219 

1.27  per  cent,  of  acid  at  59°  F.,  while  under  the  same  conditions 
B.  pasteurianum  will  produce  0.08  per  cent.  But,  more  important 
still,  is  the  fact  that  the  temperatures  which  favor  the  different 
varieties  are  not  the  same.  B.  aceti,  for  example,  produces  a  good 
fermentation  at  a  temperature  as  high  as42°F.,  whereas  B.  pasteuri- 
anum at  the  same  temperature  will  hardly  multiply,  and  produces 
no  fermentation.  Some  of  the  species  produce  the  maximum 
effect  more  quickly  than  others,  and  some  may  begin  to  destroy  the 
acid  produced  under  conditions  of  temperature  and  time  in  which 
other  varieties  are  still  active. 

Methods  of  Vinegar-making. — The  farmer's  method  of  vinegar- 
making  is  simply  to  allow  cider  to  remain  in  barrels  for  a  sufficient 
number  of  months  to  turn  into  vinegar.  The  result  of  this  method 
is  variable,  sometimes  very  good  and  sometimes  very  poor  vinegar 
resulting.  Chance  is  relied  upon  to  insure  the  presence  of  the 
proper  vinegar  organisms,  although  to  make  more  sure  of  a  good 
result,  barrels  are  sometimes  taken  that  have  previously  been  used 
for  the  same  purpose,  and  that  are,  therefore,  more  likely  to  contain 
the  desired  bacteria. 

But  vinegar- making,  like  other  farm  processes  formerly  carried 
out  by  each  farm  independently,  is  becoming  largely  locahzed  in 
factories,  where  it  can  be  conducted  on  a  large  scale  and  can  be 
more  carefully  watched.  In  these  factories  two  methods  are  used, 
differing  radically  from  each  other,  and  while  different  factories 
have  varying  details  in  the  process,  the  methods  employed  are 
modifications  of  these  two. 

The  Orleans  Process. — Oaken  casks  are  used,  each  new  cask 
being  first  steamed  and  then  impregnated  with  hot  vinegar,  to  "sour" 
the  cask.  After  this  it  is  filled  partly  full  of  good  clear  vinegar  and 
about  half  a  gallon  of  wine  is  added.  This  mixture  is  kept  at  about 
70°  F.  for  a  week  or  so,  when  a  little  more  wine  is  added,  supple- 
mented in  another  week  by  another  lot.  This  is  continued  until 
the  cask  contains  about  forty  gallons  (two-thirds  full).  Then  about 
half  of  the  material  is  withdrawn,  as  vinegar;  and  from  this  time 
on  some  two  gallons  of  vinegar  may  be  withdrawn  at  a  time,  its 
place  being  made  good  by  the  addition  of  wine.     The  cask,  when 


220     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

once  started,  may  continue  in  operation  some  six  or  eight  years  con- 
tinuously, but  eventually  it  becomes  so  filled  with  tartar  and  mother 
of  vinegar  that  it  must  be  cleansed. 

Shortly  after  the  process  of  vinegar-making  is  started  a  skin  of 
the  vinegar  organisms  grows  on  the  surface,  soon  covering  the 
whole.  Its  action  causes  the  alcohol  to  unite  with  oxygen  from  the 
air,  thus  producing  acetic  acid.  The  bacteria  in  this  method  grow 
chiefly  on  the  surface  of  the  liquid,  and  they  develop  luxuriantly 
during  the  process.  The  oxidation  does  not  always  stop  at  the 
formation  of  acetic  acid,  but  is  sometimes  carried  further  so  as  to 
split  up  the  alcohol  into  simpler  molecules.  This  results  in  a  loss, 
and  is  one  of  the  difficulties  to  be  met  with  in  the  manufacture  of  vin- 
egar. This  loss,  though  sometimes  considerable,  is  generally  not 
great,  for  the  accumulation  of  acetic  acid  will  soon  stop  the  growth 
of  the  organisms.  Different  species  of  the  organisms  can  endure 
different  amounts,  but  when  the  acetic  acid  has  reached  14  per  cent, 
the  bacteria  are  never  able  to  produce  any  more. 

The  Quick  Process. — A  second  process  of  vinegar-making,  known 
as  the  "quick  process,"  does  not,  at  first  sight,  appear  to  be  caused 
by  microorganisms.  This  process  consists  simply  in  an  intimate 
mixture  of  alcohol  with  air  by  means  of  shavings.  A  mass  of 
shavings  is  placed  in  tall  vessels  and  thoroughly  moistened  with  an 
alcoholic  solution.  Then  the  whole  is  inoculated  with  a  little  warm 
vinegar  followed  by  alcohol.  The  vinegar  thus  added  starts  the 
process,  and  in  a  few  hours  new  vinegar  is  produced.  Alcohol  is 
now  added  at  the  top,  slowly  but  continuously,  and  it  percolates 
through  the  shavings  and  appears  at  the  bottom  as  vinegar.  Such 
a  process  seems  at  first  to  be  more  like  a  chemical  phenomenon  than 
a  fermentation  induced  by  microorganisms.  But  it  does  not  start 
until  a  little  warm  vinegar  has  been  added  to  the  mixture,  and  such 
vinegar  will  be  sure  to  contain  bacteria.  This,  of  course,  suggests 
that  the  microorganisms,  thus  added,  spread  through  the  shavings, 
grow  rapidly,  and  soon  induce  the  oxidation.  Indeed,  it  has  been 
proved  that,  if  the  growth  of  the  fungi  is  prevented  in  such  a  mixture, 
no  vinegar  is  formed.  The  shavings  simply  furnish  a  large  surface 
for  the  spreading  out  of  the  organisms.     Hence,  the  quick  vinegar 


VINEGAR-MAKING.  221 

process  is  also  dependent  upon  the  presence  and  active  growth  of  the 
vinegar  plants. 

Use  of  Pure  Cultures  in  Vinegar-making. — The  different 
types  of  vinegar-making  organisms  vary  much,  requiring  quite 
different  conditions  of  temperature  for  their  best  work.  In  the 
processes  of  manufacture  hitherto  used  considerable  losses  have 
resulted  from  the  fact  that  the  acetic  acid  formed  is  destroyed  by 
the  further  action  of  the  bacteria.  The  fermentation  is  apt  to  be 
irregular  and,  even  under  the  same  conditions,  does  not  always 
produce  equally  desirable  results.  The  reasons  for  the  irregularities 
are  to  be  found  in  the  fact  that,  in  different  factories,  sometimes 
even  in  the  same  factory  at  different  times,  many  varieties  of  the 
vinegar  organisms  are  found.  Moreover,  several  of  these  organisms 
are  almost  sure  to  be  found  together  in  any  mass  of  natural  "  mother." 
It  is  impossible  to  devise  conditions  that  will  be  most  favorable  for 
all  of  the  organisms,  and  hence,  with  a  "mother"  composed  of 
many  varieties,  there  is  sure  to  be  some  loss.  Theoretically,  it 
would  seem  that  vinegar-making  could  be  improved  by  the  use 
of  pure  cultures  of  these  organisms.  With  a  pure  culture  it  would 
be  possible  to  make  the  conditions  such  as  to  produce  the  best 
results  with  the  variety  in  question,  and  thus  prevent  the  losses 
that  come  from  one  variety  destroying  the  product  of  another 
variety.  The  use  of  pure  cultures  has  revolutionized  brewing, 
has  greatly  improved  the  methods  of  butter-making,  and  is  rapidly 
changing  the  process  of  cheese-making.  Bacteriologists  feel 
confident  that  the  same  principal  can  be  apphed  to  vinegar-making. 

Hitherto  there  has  been  but  little  application  of  pure  cultures 
to  vinegar-making.  To  obtain  absolutely  pure  cultures  is  difficult, 
and  to  keep  them  pure  from  subsequent  contamination  is  more 
difficult  still.  Vinegar-makers  have  not  thought  the  advantages 
derived  from  such  methods  sufficient  to  pay  for  the  labor  and  trouble 
of  applying  them.  They  have,  to  a  considerable  extent,  adopted 
rougher  methods  of  obtaining  the  vinegar  organism  in  quantity 
and  in  a  comparatively  pure  condition.  A  little  white  spirit  vinegar 
is  filtered  through  fine  bolting  cloth  to  remove  the  vinegar  eels. 
This  is  mixed  with  a  little  alcohol  of  90  per  cent,  volume,  and  some 


222     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

white  wine  which  has  been  previously  boiled,  filtered  and  cooled. 
This  mixture  is  placed  in  shallow  dishes  and  covered  with  glass 
plates.  The  vinegar  organism  appears  in  a  few  hours  as  a  thin  scum 
which  ordinarily  will  be  pure,  or  nearly  so.  If  the  scum  does  not 
show  any  white  spots  (molds)  it  is  gently  lowered  upon  the  surface 
of  the  vat  containing  the  alcoholic  solution  which  is  to  be  made 
into  vinegar.  The  result  is  a  comparatively  pure  culture  of  vinegar 
organisms,  and  a  satisfactory  fermentation.  When  used  in  making 
vinegar  from  cider,  this  process  gives  a  vinegar  quite  superior 
to  the  ordinary  type,  having  a  finer  flavor  and  better  keeping 
properties. 

The  farmer  who  simply  lays  aside  his  few  barrels  of  cider  or 
other  alcoholic  'solution  that  it  may  be  converted  into  vinegar 
will  not  be  troubled  or  especially  interested  in  the  matter  of  pure 
cultures.  A  little  loss  is  nothing  to  him,  while  the  preparing  and 
preserving  of  pure  cultures  is  an  impossibility.  He  feels  tolerably 
confident  that  the  cider  which  he  sets  aside  for  the  purpose  will 
contain  some  of  the  acetic  acid  bacteria  and  that  in  course  of  time 
he  will  obtain  vinegar.  Whether  he  gets  the  advantage  of  all  the 
alcohol  or  loses  half  of  it  does  not  matter  much  to  him.  Even 
if  several  barrels  should  not  produce  a  proper  quality  of  vinegar, 
it  would  not  be  of  much  importance.  Sometimes  he  finds  that  his 
vinegar  is  stronger  than  at  other  times,  and  sometimes  he  finds 
that  its  taste  is  much  inferior  to  the  ordinary  grade  of  vinegar. 
Perhaps  this  raises  in  his  mind  a  temporary  question  as  to  the 
reason  for  the  differences.     But  he  never  pursues  the  subject  further. 

The  Preservation  of  Vinegar. — Vinegar  is  apt  to  deteriorate 
by  standing.  It  loses  some  of  its  acidity,  falls  off  in  flavor,  and 
may  become  muddy  and  slimy.  All  these  various  troubles  are 
caused  by  the  continued  growth  of  the  microorganisms  in  the 
vinegar.  In  such  vinegar  may  be  found  various  growing  bacteria, 
and  very  commonly,  especially  in  cider  vinegar,  may  be  found 
vinegar  eels  in  abundance.  These  latter  are  little  worms  that  get 
into  the  vinegar  from  some  source  and  find  it  a  favorable  locality 
for  growth  and  multiplication,  They  probably  injure  the  quality 
of  the  vinegar,  although,  so  far  as  is  known,  they  are  harmless 


SAUER    KRAUT.  223 

to  one  who  may  consume  the  vinegar.  Their  presence  is,  however, 
undesirable.  All  these  troubles  increase  as  the  amount  of  acid 
decreases,  for  the  miscellaneous  bacteria  and  the  vinegar  eels  cannot 
grow  if  the  acidity  is  high,  but  they  will  grow  when  this  acidity 
decreases.  The  reduction  in  acidity  is  commonly  due  to  the 
action  of  bacteria  and  usually  to  the  very  kind  of  bacteria  that 
originally  produced  it;  for  these  organisms,  after  producing  the  acid, 
may  cause  a  further  oxidation  which  destroys  it.  Hence,  it  has 
been  suggested  that  pasteurization  of  the  vinegar  for  the  purpose 
of  destroying  the  bacteria  and  vinegar  eels  will  enhance  its  keeping 
property.  Whether  there  is  any  practical  value  in  this  procedure 
has  not  yet  been  determined  by  experience. 

SAUER  KRAUT. 

This  is  a  food  used  so  widely  in  Europe  and  coming  to  be  so 
popular  in  this  country  that  it  may  well  be  regarded  as  one  of  the 
most  important  of  the  minor  farm  products.  It  is  made  of  cabbage, 
slightly  fermented  and  prevented  from  decay  by  lactic  acid  bacteria 
(Fig.  49).  The  cabbage  leaves,  after  being  washed,  are  shredded 
and  packed  in  casks  under  pressure,  to  remove  most  of  the  moisture. 
In    these   casks  a  fermentation    soon  starts,  ^ 

which   is  two-fold.      Yeasts  that  are  present  ^    ^^0 

produce  an   alcoholic  fermentation,  evolving  ^    §8 

gas  that  causes  the  v/hole  mass  to  foam  and  _  ^     9n 

froth.     At  the  same  time,  the  lactic  acid  bac-         qX    Q©  ^^3^ 
teria,  the  same  species  apparently  that  sour       Fig.  49.— The  bacteria 
milk,  develop  rapidly  and  cause  the  mass  to     and  "yeasts  that  ferment 

,  sauer  kraut. 

sour.  The  bacteria  growth  is  primarily  in 
the  juice  squeezed  out  of  the  cabbage  tissue,  and  not  in  the 
solid  matter.  As  the  mass  becomes  sour  from  the  acid,  the 
growth  of  all  bacteria  is  checked.  By  this  means  the  ordinary 
putrefactive  organisms  are  prevented  from  producing  the  decay 
of  the  vegetable  mass,  as  they  would  otherwise  do.  When  properly 
soured,  sauer  kraut  has  a  flavor  that  makes  it  a  relish.  It  may  be 
ready  for  consumption  in  two  weeks,  but  it  is  usually  kept  much 


224     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

longer  than  this.  So  long  as  it  remains  properly  acid  it  will  keep, 
and  if  kept  cold  will  remain  in  this  condition  for  many  months. 
Eventually  a  scum  appears  on  its  surface.  This  scum  proves  to 
be  made  of  microorganisms,  chiefly  a  species  called  Oidium  lactis, 
a  very  common  species  around  farms.  These  organisms,  growing 
in  the  scum,  gradually  absorb  and  destroy  the  lactic  acid  and,  as 
they  do  so,  the  sauer  kraut  becomes  less  acid  and  is  finally  alkaline. 
After  this  has  occurred  the  putrefactive  bacteria  that  are  sure  to 
be  present  have  an  opportunity  to  grow,  and  the  sauer  kraut  begins 
to  decay,  so  that  it  rapidly  spoils.  This  product  is,  thus,  one  that 
is  at  first  prepared  and  preserved  by  certain  kinds  of  microorganisms, 
but  is  eventually  ruined  by  the  growth  of  other  species. 

Certain  other  vegetables  are  prepared  in  a  similar  way.  Soured 
beans  are  prepared  in  certain  countries,  and  the  souring  of  cucum- 
bers to  make  dill-pickles  is,  apparently,  an  identical  process.  Soured 
beets  and  asparagus  are  also  articles  of  diet.  Bacteria  similar  to 
those  found  in  sauer  kraut  are  concerned  both  in  the  souring  of 
these  products  and  in  the  subsequent  neutralization  of  the  acid 
preparatory  to  the  final  spoiling  of  the  product. 

THE  CURING  OF  TOBACCO. 

Tobacco  is  a  product  whose  value  is  almost  wholly  dependent 
upon  the  success  of  its  curing  and  its  final  preparation  for  market. 
The  green  plant,  as  taken  from  the  field,  is  in  itself  valueless,  and 
many  a  crop  is  injured  or  perhaps  ruined  in  the  curing.  The 
relation  of  the  curing  to  microorganisms  is  not  yet  settled,  but 
since  the  curing  is  undoubtedly  a  fermentation,  it  properly  belongs 
to  our  subject. 

When  the  leaves  are  fully  grown  the  crop  is  reaped  and  hung  up 
in  a  shed  or  barn  to  undergo  a  partial  drying.  After  the  drying 
process  has  reached  a  desired  stage  the  leaves  are  ready  for  the 
fermentation  proper,  the  process  upon  which  the  value  of  the 
product  largely  depends.  There  are  two  quite  different  methods 
of  bringing  about  this  fermentation.  In  the  first  method  the  leaves 
are  left  hanging  a  long  time,  and  are  eventually  packed  closely  in 


THE    CURING    OF    TOBACCO.  225 

boxes  weighing  several  hundreds  of  pounds  each.  These  boxes  are 
then  left  to  take  care  of  themselves.  They  are  generally  packed  in 
the  cool  weather  of  fall  and  remain  undisturbed  several  months. 
When  the  warmer  weather  comes,  in  the  spring,  a  fermentation  is  set 
up  in  the  cases,  which  progresses  without  any  attention  from  the 
owner;  but  after  a  number  of  months  the  boxes  are  opened  to  de- 
termine the  success  of  the  fermentation,  and  the  crop  is  sold  at  a 
price  depending  upon  the  character  of  the  product. 

The  second  method  of  fermentation,  adopted  chiefly  in  warmer 
climates,  keeps  the  whole  process  under  close  observation  and  is,  in 
this  respect,  undoubtedly  superior.  The  leaves,  after  drying,  are 
piled  upon  each  other,  not  too  tightly,  and  a  great  heap  is  made, 
sometimes  three  feet  high,  sometimes  more.  For  the  proper  fer- 
mentation of  this  heap  there  should  be  a  warm,  moist  atmosphere, 
such  as  is  found  in  tropical  and  semi-tropical  climates.  Within  a 
short  time  the  temperature  of  these  masses  begins  to  rise,  sometimes 
as  high  as  ten  degrees  in  a  day.  When  the  temperature  reaches  a 
point  between  125°  and  130°  F.,  the  piles  are  opened  and  the  leaves 
are  heaped  up  again  in  other  similar  piles,  care  being  taken  to  put 
on  the  inside  those  leaves  which  were  before  on  the  outside. 
Another  rise  in  temperature  follows  and  again,  after  reaching  i25°F., 
the  heaps  are  thrown  down  and  remade.  This  is  repeated  from 
five  to  eight  times,  several  days  elapsing  between  the  successive 
heapings.  At  the  end  the  tobacco  is  in  the  proper  condition  for 
market.  This  second  method  is  quicker  and,  in  some  respects, 
better  than  the  first  method.  The  fermentations  do  not  always  end 
here,  however.  The  manufacturer  commonly  allows  the  tobacco 
to  undergo  a  second  fermentation,  called  "sweating,"  which  brings 
the  leaf  into  a  better  condition  for  use. 

The  primary  fermentation  is  clearly  the  essential  process  of 
tobacco-curing.  During  the  fermentation  some  very  essential 
changes  in  the  tobacco  take  place.  The  chief  of  these  changes  are 
the  following:  A  decrease  in  nicotin,  an  increase  in  alkaline  reaction, 
an  increase  in  ammonia,  the  disappearance  of  sugar,  an  increase  in 
the  amount  of  nitrate,  a  loss  of  water,  a  change  in  the  texture  of 
the  leaf,  a  change  in  color  (the  final  color  brown)  and  a  change  in 


226     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

flavor.  These  numerous  changes,  while  quite  varied  in  nature,  are 
mainly  due  to  oxidation. 

The  Cause  of  Tobacco  Fermentation. — Three  different 
theories  have  been  held  and,  to  a  certain  extent,  are  still  held,  con- 
cerning the  cause  of  the  fermentation.  While  not  one  of  them 
explains  all  the  facts,  all  three  may,  in  a  measure,  be  correct. 

The  Chemical  Theory. — This  assumes  that  the  free  oxygen  of 
the  air  acts  directly  upon  the  cells  of  the  tobacco  leaves,  although  it 
is  admitted  that  microorganisms  are  necessary  to  raise  the  tem- 
perature sufficiently  to  make  the  oxidation  possible. 

The  Bacterial  Theory. — This  assumes  that  the  fermentation  is 
the  result  of  bacterial  growth.  Bacteria  are  found  upon  the  leaves 
of  fermenting  tobacco,  and  several  distinct  species  have  been 
isolated  and  carefully  studied.  Some  of  them  are  well-known 
species,  but  others  are  peculiar  to  tobacco.  Some  have  been 
named  B.  tohacci  /,  //,  III,  IV,  and  V.  It  has  been  claimed  that 
these  have  a  causal  relation  to  the  fermentation  of  the  tobacco,  and 
experiments  have  been  carried  out  to  test  it.  Tobacco  leaves  have 
been  sterilized  and  it  is  found  that  they  will  not  undergo  fermenta- 
tion. If,  however,  they  are  inoculated  with  some  of  these  bacteria, 
they  will  undergo  a  fermentation,  although  the  result  does  not  show  a 
good  flavor  or  aroma,  which  fact  suggests  that,  though  bacteria  are 
concerned  in  the  process,  there  are  other  factors. 

The  Enzyme  Theory. — The  recognized  importance  of  enzymes 
in  fermentation  in  general,  has  naturally  raised  the  question  of 
their  relation  to  tobacco-curing.  Enzymes  are  known  to  be  pro- 
duced by  plants,  and  would  be  expected  in  the  tobacco  leaves. 
Indeed,  they  are  found  there  readily  enough,  and  among  them  are 
certain  enzymes  called  ozydases,  peroxydases,  and  catalases,  which 
have  the  power,  under  different  conditions,  of  producing  an  oxida- 
tion of  other  substances.  Are  not  these,  rather  than  bacteria,  the 
cause  of  tobacco  fermentation,  which  is  chemically  an  oxidation? 
Arguments  for  this  view  are  found  in  the  following  facts: 

I.  The  extremely  rapid  rise  in  temperature  is  too  high  to  be 
accounted  for  by  ordinary  bacterial  action.  Fermentation  due  to 
bacteria  may  certainly  produce  a  rise  in  temperature,  but  a  rise  as 


THE    CURING    OF    TOBACCO.  227 

high  as  130°  F.  is  entirely  beyond  anything  that  could  be  expected 
of  living  microorganisms.  2.  The  fermentation  will  go  on  in  the 
presence  of  corrosive  sublimate  that  prevents  bacteria  growth. 
3.  While  bacteria  may  be  found  upon  the  leaves  of  the  fermenting 
tobacco  they  are  generally  found  only  in  small  quantities,  too  few 
to  account  for  the  fermentation  which  is  producing  a  rise  in 
temperature  of  io°  per  day.  Moreover,  the  amount  of  moisture  in 
the  tobacco  leaves  is  low,  not  over  25  per  cent.,  and  in  such  a  condi- 
tion bacteria  do  not  readily  grow.  Lastly,  nicotine  is  generally 
looked  upon  as  a  means  of  checking  bacteria,  and  hence  the  ferment- 
ing tobacco  cannot  be  regarded  as  a  favorable  place  for  bacteria 
growth. 

On  the  other  hand,  these  enzymes  are  found  in  abundance  on  the 
leaves,  and  they  are  capable  of  producing  an  oxidation,  such  as 
occurs  during  the  fermentation.  The  conclusion  that  the  enzymes 
from  the  tobacco  leaves  are  active  agents  in  the  curing  seems 
indisputable. 

But  while  these  facts  suggest  that  enzymes  may  play  the  chief 
part  in  the  fermentation,  they  by  no  means  exclude  the  action  of 
bacteria.  Tobacco  lovers  know  that  the  tobacco  of  Cuba  develops 
in  its  fermentation  a  flavor  which  is  not  found  in  tobacco  prepared 
elsewhere.  The  same  species  of  tobacco  raised  in  other  countries, 
although  it  will  undergo  a  fermentation  of  a  normal  character, 
acquiring  the  chemical  and  physical  properties  which  it  develops 
in  Cuba,  does  not  acquire  the  flavor  that  it  has  in  its  own  home. 
Cuban  tobacco  is  now  raised  in  the  United  States,  but  its  flavor 
is  inferior  to  that  raised  in  Cuba. 

We  have  already  noticed  that  in  the  ripening  of  cheese,  though 
the  enzymes  are  extremely  important  agents  in  the  chemical  changes 
going  on,  the  bacteria  are  of  chief  importance  in  the  production  of 
the  flavors.  The  fermentation  of  the  tobacco  by  the  oxydases  does 
not  satisfactorily  explain  the  flavors.  When  Havana  tobacco 
is  fermented  in  the  United  States,  it  ferments  normally,  but  does 
not  develop  the  typical  Cuban  flavor.  It  is  quite  possible  that 
this  flavor  is,  after  all,  a  matter  of  bacterial  action.  When  the 
Cuban  planter  ferments  his  tobacco,   he  commonly  sprinkles  it 


228     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

with  special  preparations,  a  process  called  "petuning."  These 
preparations  are  usually  secrets,  and  each  plantation  is  likely  to 
have  its  own.  They  consist  of  mixtures  of  various  chemicals,  of 
which  organic  fluids  containing  ammonium  carbonate  frequently 
form  a  part.  The  action  of  this  petuning  is  problematical,  but  it 
is  believed  by  the  planters  to  contribute  to  the  production  of  the 
peculiar  flavors  of  Cuban  tobacco. 

Now  these  mixtures  are  good  culture  media  for  bacteria,  and 
when  they  are  sprinkled  upon  the  tobacco  the  leaves  are,  in  a  way, 
inoculated  with  bacteria.  On  such  petuned  leaves  bacteria  are 
abundant.  But  there  is  no  evidence  at  hand  to  indicate  whether 
these  bacteria  have  anything  to  do  with  the  production  of  flavor. 
It  is  certainly  not  impossible  nor  improbable  that  the  flavor 
production,  which  does  not  seem  to  appear  typically  outside  of 
Cuba,  may  be  due  in  part  to  bacterial  action,  possibly  to  the  action 
of  the  very  bacteria  that  the  planter  unconsciously  sprinkles  over  his 
leaves  in  the  petuning  which  occurs  before  the  fermentation  begins. 

Of  course  the  suggestion  that  the  flavor  of  tobacco  may  be 
improved  by  the  use  of  artificial  pure  cultures  in  the  fermentation 
is  a  natural  one.  The  acknowledged  relations  of  bacteria  to  the 
flavors  of  butter,  cheese,  and  other  products  naturally  suggest  an 
attempt  to  improve  the  flavors  of  tobacco  by  bacterial  inoculation. 
Several  experimenters  have  been  trying  this  plan  for  years,  with 
what  success  it  is  hardly  possible  to  say.  The  manifest  financial 
importance  of  such  a  process,  could  it  be  made  successful,  has 
inclined  experimenters  to  keep  their  work  secret.  While  it  has 
several  times  been  claimed  that  by  the  use  of  proper  bacterial 
cultures  Havana  flavors  can  be  obtained  in  tobacco  fermented 
elsewhere,  these  claims  have  not  yet  been  substantiated  by  any 
public  demonstration.  They  are  still  made  by  some  who  insist 
that  they  have  actually  been  successful  in  this  line  of  experimenting 
and  that  they  have  made  Havana  flavors  from  common  tobacco 
by  bacterial  inoculations. 

Diseases  of  Tobacco. — Whether  or  not  microorganisms  play 
a  part  in  the  normal  ripening,  it  is  certain  that  they  sometimes  injure 
the  crop  and  produce  abnormal  fermentation.     The  presence  of 


SILAGE. 


229 


too  much  moisture  on  the  leaves  is  likely  to  be  followed  by  the  growth 
of  mischievous  microorganisms.  Molds  are  the  most  common 
injurious  organisms  to  appear  under  these  conditions,  but  bacteria 
may  also  develop  and  produce  disastrous  results.  From  the  time 
the  tobacco  begins  to  grow  in  the  field  until  it  has  reached  its  final 
state  as  a  completed  product,  it  is  subject  to  a  considerable  number 
of  diseases.  It  is  a  very  delicate  plant,  and  slight  changes  in 
moisture  or  temperature  are  almost  sure  to  bring  about  troubles  of 
some  kind  that  injure  or  ruin  the  crop.  Of  these  troubles  some  are 
produced  by  molds  or  special  fungi,  and  some  by  bacteria.  The 
consideration  of  these  various  troubles,  whether  bacteiial  or  of  a 
different  nature,  concerns  only  the  person  interested  in  raising  tobacco 
and  is  of  no  special  interest  to  the  agriculturist  in  general.  We 
shall  not,  therefore,  further  consider  them  in  this  work. 

SILAGE. 

In  the  silo  the  agriculturist  has  devised  a  method  of  utilizing 
certain  food  products  of  which  the  soil  yields  large  crops,  but  which 
contain  so  much  water  that  they  lose  their  value  in  great  measure 
when  dried.  The  silo  not  only  enables  him  to  preserve  such  food, 
but  it  impregnates  it  with  new  flavors  which,  in  some  respects,  en- 
hance its  value,  for  it  makes  a  product  especially  relished  by  cattle. 

Preparation. — In  the  preparation  of  silage  the  material  to  be 
used,  most  commonly  corn  not  fully  ripe,  is  cut  into  moderately 
small  pieces  and  packed  away  firmly  as  a  solid  mass,  in  a  tall,  air- 
tight compartment.  Sometimes  the  silo  is  filled  quickly,  and  some- 
times more  slowly,  and  the  rapidity  should  depend  upon  the  rapidity 
of  fermentation.  After  the  silo  is  filled  it  is  closed  at  the  top,  and 
frequently  subjected  to  considerable  pressure.  The  contents  are, 
thus,  largely  deprived  of  air.  Air,  of  course,  gets  in  around  the 
top,  but  there  is  little  or  none  around  the  sides  or  bottom,  so  that 
only  the  superficial  layers  are  affected  by  it. 

Fermentation. — After  the  packing  important  and  profound 
changes  take  place  in  the  silage.  The  first  phenomenon  to  be 
noticed  is  a  rapid  rise  in  temperature,  the  primary  fermentation. 


230     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

The  extent  of  this  rise  is  dependent  upon  the  amount  of  oxygen 
present  and  the  readiness  with  which  the  heat  is  radiated.  The 
temperature  should  not  rise  above  150°  F.,  and,  to  give  the  best 
result,  it  should  be  much  lower.  The  proper  production  of  silage, 
however,  does  not  appear  to  be  dependent  upon  this  rise  in  tempera- 
ture, inasmuch  as  perfectly  normal  silage  may  be  made  in  small 
vessels  where  hardly  any  rise  in  temperature  is  noticeable. 

This  high  temperature  lasts  a  few  days  and  then  the  mass  slowly 
cools.  The  production  of  heat  appears  to  be  very  rapid  for  a  few 
days,  and  then  somewhat  quickly  declines;  but  a  less  rapid  evolution 
of  heat  continues  for  a  long  time,  perhaps  several  weeks.  After 
the  reduction  in  temperature  other  changes  begin,  which  are  much 
slower,  and  after  several  weeks  the  character  of  the  material  is 
found  to  be  greatly  changed.  This  is  a  secondary  fermentation. 
of  a  different  type.  It  develops  a  certain  amount  of  acid,  its  chemical 
nature  is  altered,  and  it  develops  a  new  flavor  and  aroma  which  should 
be  distinctly  aromatic,  without  any  signs  of  putrefaction  or  mustiness. 
There  is  found  to  be  a  considerable  loss  of  material,  a  loss  ranging 
from  4  per  cent:  to  40  per  cent.  This  is  a  very  wide  range,  and 
shows  that  the  method  of  ensilage  has  an  extraordinary  effect  upon 
the  product  obtained.  The  loss  is  chiefly  a  loss  of  carbohydrates, 
although  there  is  also  an  appreciable  loss  of  albuminoids.  The 
loss  is  largely  parallel  to  the  amount  of  oxygen  that  finds  its  way 
into  the  silo,  being  very  slight  if  the  oxygen  of  the  air  be  thoroughly 
excluded.  Perfect  exclusion  of  air  is,  then,  the  best  means  of 
preventing  the  loss. 

In  a  properly  prepared  silo  the  fermentative  changes  do  not  ex- 
tend beyond  this,  and  the  material  will  now  remain  sweet  for 
months.  The  superficial  layers  may  become  decayed  and  ruined, 
but  the  central  mass  itself  is  not  affected.  After  the  feeding  from  a 
silo  is  commenced  its  contents  must  be  used  up  somewhat  rapidly, 
for  various  undesirable  fermentative  changes  may  set  up  in  the 
superficial  layers  as  they  are  successively  exposed  to  the  air. 

The  Causes  of  Ensilage  Fermentation. — Three  different 
factors  have  been  suggested  as  causes  for  the  fermentations  inside 
the   silo.     These   are:  i.    The   action   of  bacteria.     2.  Respiratory 


SILAGE.  231 

changes  in  the  plant  tissues,  and  3.  The  action  of  enzymes.  The 
probabihty  is  that,  as  in  the  other  cases  where  there  has  been  a 
similar  dispute,  all  three  processes  are  concerned. 

Respiratory  Changes. — The  living  plant  cell  is  always  carrying 
on  the  physiological  process  of  respiration,  a  process  quite  similar 
to  respiration  in  animals,  and  resulting  in  the  use  of  oxygen  and  the 
evolution  of  carbon  dioxid.  In  this  respiration  carbohydrate 
bodies  are  used,  with  some  albuminoids  as  well,  and  heat  is  evolved. 
Now,  the  plant  cells  do  not  die  when  the  plant  is  cut  down,  but  con- 
tinue for  some  considerable  time  to  carry  on  this  process  of  respira- 
tion. Cutting  the  plant  to  pieces  appears,  indeed,  to  increase 
temporarily  rather  than  to  decrease  the  respiratory  changes. 
These  may  go  on  for  several  days,  until,  indeed,  the  plant  cells  are 
fully  dead.  These  are  well-known  facts,  and  recognized  by  botanists 
for  a  long  time.  To  these  respiratory  changes  is  due  part  of  the 
fermentation  in  silage.  After  the  material  is  packed  in  the  silo  the 
plant  cells  remain  alive  for  several  days  and  carry  on  these  respira- 
tory changes  as  long  as  they  are  alive  and  have  oxygen  at  their  com- 
mand. This  results  in  the  gradual  oxidation  of  the  carbohydrate 
material  and  the  evolution  of  carbon  dioxid.  These  changes  are 
thought  to  be  fully  sufl&cient  to  explain  the  first  changes  in  the 
silage,  with  the  initial  heating  and  evolution  of  gas. 

Fermentations  Due  to  Enzymes. — As  already  noticed,  living  plant 
tissues  secrete  a  variety  of  enzymes  with  varying  powers  of  acting 
upon  carbohydrates  and  albuminoids.  Such  enzymes  are  present 
in  the  corn  stalks  and  fruit,  and  when  these  are  packed  in  the  silo, 
the  enzymes  are  of  course  stowed  away  with  them.  As  the  mass  is 
warmed  up  under  the  action  of  the  respiratory  process  it  is  inevitable 
that  the  enzymes  will  begin  their  action,  and  that  the  fermentations 
occurring  during  the  next  few  weeks  will  be  affected  by  these 
enzyme  activities.  It  has  as  yet  been  impossible  to  say  to  what 
extent  the  enzyme  action  is  concerned  in  the  phenomenon.  Certainly 
they  must  have  much  to  do  with  the  result. 

The  respiratory  processes  and  the  action  of  the  enzymes  to- 
gether are  capable  of  producing  silage  of  ordinary  type  without  the 
aid  of  bacteria  or  other  living  agencies.     Silage  can  be  made  in 


232     ALCOHOL,  VINEGAR,  SAUER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

experimental  jars  in  which  chloroform  vapor  prevents  the  growth  of 
bacteria,  although  it  allows  the  enzyme  action  to  continue  as  usual. 
It  goes  through  a  fermentation  that  is  fairly  typical,  a  fact  that  shows 
that  the  essential  phenomena  of  ensiling  may  be  wholly  the  result  of 
these  two  sets  of  activities. 

The  Action  of  Microorganisms. — It  was  first  thought  that  the 
fermentation  of  silage  was  a  bacterial  action  wholly,  but  further 
study  showed  the  fallacy  of  this  conception.  The  original  fer- 
mentation, by  which  there  is  a  rapid  rise  in  temperature,  cannot 
be  the  result  of  bacterial  growth,  since  it  is  too  rapid  and  the 
temperature  rises  too  high.  There  is  no  evidence  to  suggest  that 
any  bacteria  can  produce  a  rise  as  high  as  150°,  a  temperature  that 
destroys  the  life  of  most  organisms.  If  the  rise  were  due  to  bacteria 
it  would  be  rather  slow,  rising  only  as  the  bacteria  had  the  oppor- 
tunity to  develop,  while  on  the  contrary  it  is  very  rapid.  Finally 
the  possibility  of  making  silage  in  a  jar  filled  with  chloroform  vapor 
shows  that  bacteria  are  not  necessary  for  the  phenomenon. 

But  this  does  not  by  any  means  exclude  the  agency  of  micro- 
organisms in  the  ordinary  formation  of  silage.  Bacteria  are  cer- 
tainly present  and,  in  some  cases,  they  are  present  in  great  numbers. 
Some  bacteriologists  have  not  been  able  to  find  them  so  very  abun- 
dantly, but  this  seems  to  be  due  to  the  fact  that  they  did  not  use 
favorable  media  in  studying  them,  for  when  a  medium  is  used 
that  is  adapted  to  the  silage  bacteria,  they  are  found  in  abundance. 
Certainly,  if  they  are  present  and  develop  during  the  fermentation, 
they  must  have  some  effect  upon  the  silage.  One  effect  they  cer- 
tainly seem  to  have.  The  silage  turns  acid  during  the  ensiling,  and 
this  acidity  appears  here,  as  in  other  cases  which  we  have  noticed,  to  be 
a  means  of  preventing  subsequent  putrefactive  changes.  Without 
doubt  this  acidity  may  be  attributed  in  part,  if  not  wholly,  to  acid- 
forming  bacteria  growing  in  the  silo. 

Further,  there  develop  in  the  silage  certain  prominent  flavors 
which  contribute  largely  to  its  value,  and  the  source  of  these  flavors 
is  as  yet  unknown.  Aromatic  flavors  such  as  are  found  in  silage  do 
not  come  from  respiratory  processes,  nor  do  enzymes  develop  such 
flavors,  so  far  as  is  known.     There  are,  some  who  think,  however, 


SOUR    FODDER.  233 

that  silage  flavors  are  really  due  to  enzyme  action.  But  considering 
the  fact  that  enzymes  do  not  commonly  produce  any  such  flavors 
while  bacteria  do,  and  also  that  bacteria  certainly  grow  in  the  silage 
after  the  first  fermentation  is  over,  it  seems  on  the  whole  more 
likely  that  the  flavors  must  be  attributed  to  bacterial  action. 

As  a  summary,  then,  it  appears  that  silage  involves  three  distinct 
processes,  each  of  which  is  capable  of  producing  a  profound  modifi- 
cation of  the  material  in  the  silo.  Probably  all  three  may  be  con- 
cerned, in  different  degrees^  at  different  times.  The  various  lots 
of  silage  do  not  always  ferment  alike,  even  under  seemingly  identical 
conditions;  and  very  possibly  these  three  different  processes  are  con- 
cerned, in  varying  degrees,  in  the  different  lots  of  silage.  The  sub- 
ject is  complicated  and  probably  so  variable  that  we  cannot  at 
present  say,  with  any  degree  of  accuracy,  just  what  is  the  usual 
course  of  events  in  this  fermentation.  A  large  amount  of  study 
remains  to  be  done  on  this  subject,  and  doubtless,  when  the  matter 
is  properly  studied,  so  that  it  is  better  understood,  great  improve- 
ments can  be  made  in  the  process. 

It  is  perhaps  fitting  to  say  that  silage  forms  a  good  food  for 
cattle,  although  some  dairy  companies  refuse  to  accept  milk  from 
silage-fed  cows.  The  reason  that  this  kind  of  fodder  has  an  effect 
on  the  milk  is  probably  due  to  the  dirt  and  filth  that  get  into  the 
milk  from  the  silage  food  after  milking,  rather  than  to  the  silage 
that  the  animals  have  actually  eaten.  If  the  milk  were  kept  clean 
and  all  the  dairy  processes  carried  out  in  a  proper  manner,  it  is 
doubtful  whether  any  trace  of  silage  feeding  would  show  in  the  milk. 
But  considering  the  carelessness  in  the  ordinary  dairy  and  the  dirt 
that  commonly  gets  into  milk,  it  may  follow  that  the  effect  of  silage 
in  the  stable  will  show  in  the  milk. 

SOUR  FODDER. 

This  is  a  food  for  cattle  made  out  of  the  waste  from  beet  sugar 
manufactories,  and  other  waste  material.  Slices  of  beet  roots,  after 
the  sugar  is  extracted,  steamed  potatoes,  corn  stalks,  and  various 
other  vegetable  substances,  may  serve,  as  its  basis.     This  material 


234     ALCOHOL,  VINEGAR,  siuER  KRAUT,  TOBACCO,  SILAGE,  FLAX. 

is  packed  in  trenches  in  the  ground,  pressed  by  heavy  weights, 
and  left  to  ferment.  It  undergoes  a  fermentation  that  converts 
it  into  an  acceptable  food  for  cattle.  Its  value  lies  in  the  fact  that 
it  is  a  means  of  utilizing  what  would  otherwise  be  a  waste  product. 
It  is  of  little  significance  in  this  country. 

THE  RETTING  OF  FLAX  AND  HEMP. 

Linen  is  made  from  the  long  tough  fibers  that  are  found  beneath 
the  bark  of  the  flax  plant.  In  the  plant  they  are  firmly  bound 
together,  and  with  the  wood  and  bark  make  a  solid  mass,  glued 
together  by  a  substance  called  pectin.  To  remove  these  fibers  so 
that  they  may  be  woven  into  hnen,  this  pectin  must  be  disposed  of 
in  some  way.  The  method  by  which  this  has  been  accomplished 
from  time  immemorial  is  by  "retting."  The  flax  is  tied  up  in 
bundles  and  immersed  in  the  water  of  a  stream  or  in  vats.  Here 
it  remains  until  the  water  bacteria  have  pretty  thoroughly  rotted 
or  "retted"  it.  By  the  decomposing  action  of  these  bacteria, 
the  pectin  is  dissolved  and  the  fibers  in  the  flax  stem  are  loosened 
from  their  connection  with  the  other  parts  of  the  plant.  A  little 
combing  over  properly  constructed  teeth  separates  the  fibers  from 
each  other,  and  gives  the  desired  product  for  spinning  and  weaving. 
The  separation  of  the  flax  fibers  has  practically  always  been  done 
in  this  way.  The  bacteria  concerned  have  been  isolated  from  the 
retting  flax  and  obtained  in  pure  cultures,  and  it  is  found  that  they 
are  able  to  produce  the  result  when  inoculated  upon  flax  in  pure 
culture.  Hitherto  no  substitute  for  this  bacterial  action  has  been 
found  that  will  satisfactorily  replace  the  natural  retting.  It  is 
quite  possible  that  some  chemical  means  may  be  found  that 
will  replace  the  bacterial  process.  Indeed,  certain  secret  processes 
are  now  in  use  that  are  based  upon  chemical  methods  and  are 
claimed  to  give  uniform  results  in  a  much  shorter  time  than  the 
ordinary  retting.  How  soon  these  may  replace  the  agency  of  bacteria 
in  the  linen  industries  cannot  be  predicted. 

Hemp  is  prepared  from  the  hemp  plant  by  a  means  essentially 
similar  to  the  retting  of  flax. 


CHAPTER  XVI. 
THE  PRESERVATION  OF  FOOD  PRODUCTS. 

THE  SPOILING  OF  FOOD. 

Practically  all  the  materials  raised  on  the  farm  as  food  for 
man  or  animals  may  serve  also  as  food  for  microorganisms.  If 
any  of  the  various  fungi  attack  foods,  their  presence  is  shortly 
made  manifest  by  signs  of  decomposition.  The  food,  we  say, 
begins  to  spoil.  Any  one  of  our  food  products  that  contains  sufficient 
water  is  sure  to  be  attacked  sooner  or  later  by  some  of  the  various 
types  of  microorganisms,  especially  by  the  three  classes  we  have 
recognized. 

By  Bacteria. — These  are  particularly  adapted  for  feeding  upon 
proteid  matter,  but  they  are  not  fond  of  sugar.  Hence  we  find 
them  especially  concerned  in  the  spoiling  of  proteid  foods.  Meats, 
milk,  eggs,  wheat,  and  other  cereals,  all  contain  unusually  large 
quantities  of  proteids  and  are  liable  to  putrefaction,  whenever 
they  are  moist  enough;  and  their  putrefaction  will  practically 
always  be  found  to  be  due  to  bacteria.  In  all  attempts  to  preserve 
these  substances  it  is  to  be  remembered  that  we  are  dealing  with 
bacteria,  some  of  which  are  liable  to  form  spores  and  for  that  reason 
are  difficult  to  destroy.  Pure  sugar  solutions,  on  the  other  hand, 
will  not  undergo  a  bacterial  fermentation,  although  impure  sugars 
may  do  so. 

By  Yeasts. — Although  sugars  are  not  attacked  by  bacteria, 
they  are  the  favorite  food  of  yeasts,  which  destroy  them  by  setting 
up  an  alcoholic  fermentation.  If  the  sugar  is  in  considerable 
abundance,  it  serves  as  a  partial  protection  against  bacterial  growth^ 
although  it  favors  yeast  activities.  From  this  it  follows  that  fruit 
juices  in  particular  are  subject  to  yeast  fermentation,  but  are  not 
specially  liable  to  bacterial  action.     All  such  substances  as  fruit 

235 


236  THE    PRESERVATION    OF    FOOD    PRODUCTS. 

juices,  jellies,  jams,  in  short,  anything  preserved  by  sugar,  are 
liable  to  yeast  action.  Therefore,  in  their  preservation,  it  is 
to  be  borne  in  mind  that  we  are  dealing  chiefly  with  yeasts  which 
are  much  more  easily  killed  than  bacteria.  Sterilization  of  these 
products  is  much  easier  than  sterilization  of  proteid  foods. 

By  Molds  and  Higher  Fungi. — Although  these  are  less  impor- 
tant agents  in  the  spoiling  of  foods  than  bacteria,  they  are  important 
in  several  directions.  Many  kinds  of  food — bread,  cheese,  etc. — 
will  support  a  mold  growth  if  kept  rather  moist.  Molds  grow 
chiefly  on  the  surface,  but  when  they  become  luxuriant  they  cause 
the  material  to  become  "musty"  and  to  develop  unusual  as  well 
as  unpleasant  flavors.  Almost  any  food  might  in  time  be  completely 
spoiled  by  molds,  but  usually  the  bacteria  and  yeasts  act  more 
rapidly  than  the  molds,  so  that  mold  action  is  secondary.  In  the 
decay  of  wood  and  timber  it  is  the  higher  fungi  that  play  the  chief 
part  (bracket  fungi  and  other  tree  fungi) .  They  force  their  mycelia 
into  the  trunk  of  the  solid  tree,  softening  it  and  beginning  the 
process  of  decay.  The  common  molds  are  the  primary  cause  of 
the  decay  of  fruit,  for  they  force  their  mycelia  through  breaks 
in  the  skin  of  the  fruit  and  then  through  the  whole  fruit.  While 
yeasts  and  bacteria  may  sometimes  be  concerned  in  the  rotting  of 
fruit  the  molds  are  almost  universally  the  cause  of  this  phenomenon. 

PRESERVATION  OF  FOODS. 

The  extremely  varied  nature  of  farm  products  has  made  it 
necessary  to  find  many  different  methods  of  preservation,  since 
what  is  well  adapted  for  one  may  be  useless  for  another.  The 
method  of  preserving  wheat,  for  example,  is  not  adapted  for  pre- 
serving milk  or  fruit.  There  are  several  fundamental  methods  in 
use,  each  of  which  has  numerous  modifications. 

Protection  from  Microorganisms. — If  it  were  possible  to  pre- 
vent bacteria,  yeasts,  and  molds  from  gaining  access  to  food  materials, 
^e  food  could  be  preserved  indefinitely.  But  these  organisms  or 
their  spores  are  so  abundant  everywhere  that  this  is  impossible, 
except  by  hermetical  sealing.  Some  foods,  however,  are  thus  pro- 
tected.   Fruits  have  a  certain  amount  of  protection  against  the 


PRESERVATION    BY    DRYING.  237 

molds  that  cause  their  decay,  since  their  uninjured  skins  resist  their 
entrance.  The  smooth  hard  skin  of  many  fruits  is  impervious  to 
the  mycelium  of  the  mold,  though  it  can  readily  force  its  way  in 
through  a  bruise  or  crack  into  the  softer  substance  within.  Hence 
the  bruised  apple  decays  quickly.  Wiping  the  skin  of  fruit  clean 
and  dry  will  protect  it  for  a  long  time  from  decay.  The  wiping 
cleans  off  most  of  the  mold  spores  that  may  be  on  the  skin,  and  the 
drying  of  the  skin  leaves  no  moisture  in  which  the  few  spores  left 
can  germinate.  If  moisture  condenses  on  the  skin,  as  when  the 
fruit  is  taken  from  a  cold  room  into  a  warm  one,  decay  is  sure  to 
follow,  since  this  moisture  starts  the  germination  of  the  spores  into  a 
mycelium  and  the  latter  is  pretty  sure  to  find  some  place  in  the  skin 
through  which  it  can  pass.  Once  inside  the  skin,  it  grows  rapidly 
through  the  soft  pulp  and  the  fruit  is  soon  spoiled.  The  preserva- 
tion of  fruit  is,  thus,  a  matter  of  keeping  it  dry,  at  a  low  temperature, 
and  with  an  unbroken  skin.  Even  the  wrapping  of  fruit  in  paper 
materially  aids  in  its  keeping,  since  the  paper  absorbs  the  moisture 
that  collects  on  the  skin. 

PRESERVATION  BY  DRYING. 

The  simplest  means  of  preventing  the  growth  of  bacteria  in  food 
products  is  by  drying.  Anything  that  can  be  dried  without  destroy- 
ing its  value  as  a  food  can  in  this  way  be  effectually  protected 
against  bacterial  action.  No  method  of  preserving  food  products  is 
so  universally  used  as  this,  and  none  other  is  so  effective. 

Grains. — In  the  preservation  of  the  valuable  cereal  products 
nature  herself  adopts  this  plan  and,  when  the  grain  is  ripening,  the 
large  amount  of  water  which  was  present  in  the  green  seed  disap- 
pears, leaving  the  ripened  grain,  somewhat  shriveled,  perhaps,  but 
with  a  very  small  water  content.  Such  dried  grains  not  only  refuse 
to  germinate  unless  moistened  with  water,  but  bacteria  are  utterly 
unable  to  grow  within  them.  Nature  wishes  to  preserve  the  grain 
through  the  season  of  rest  (winter),  and  in  order  to  protect  it  from 
bacteria  she  takes  most  of  the  water  out,  thus  preventing  the  putre- 
faction which  would  otherwise  surely  take  place.     In  harvesting 


238         THE  PRESERVATION  OF  FOOD  PRODUCTS. 

the  grain,  therefore,  all  that  is  necessary  for  the  farmer  to  do  is  to 
collect  the  product  after  it  is  fully  ripened,  confident  that  it  will  not 
contain  enough  water  to  make  bacterial  growth  possible. 

Flesh. — With  other  foods  the  task  is  more  difficult.  The  flesh 
of  animals  contains  so  much  water  that  it  undergoes  decay  at  very 
short  notice.  So  abundant  are  the  bacteria  on  every  side  that  the 
drying  of  flesh  by  simple  means  is  practically  impossible.  We 
sometimes  read  of  hunters  in  the  wilds  of  nature,  or  of  savages  in 
cooler  climates  where  the  air  is  clear  and  dry,  preserving  flesh  by  the 
simple  process  of  cutting  it  into  thin  strips  and  hanging  it  up  in  the 
sun  to  dry.  Such  a  process  would  hardly  suffice  upon  an  ordinary 
farm,  for  the  flesh  would  be  sure  to  decay  before  it  was  dry  enough  to 
resist  the  action  of  bacteria.  Whether  this  is  due  to  the  greater 
amount  of  moisture  in  the  air,  or  to  the  fact  that  there  is  a  larger 
number  of  bacteria  in  the  air,  around  civilized  communities,  cannot 
be  stated.  But  it  is  certain  that  such  a  simple  method  of  drying 
flesh  cannot  be  adopted  by  farms  in  general.  This  method  of  pre- 
serving is,  however,  still  used  in  hot  climates,  commonly  with  the 
addition  of  salting,  and  produces  a  form  of  food  known  as  pemmican, 
charque,  and  tassajo.  The  flesh  thus  prepared  loses  considerable 
of  its  flavor,  but  methods  of  using  artificial  heat  have  been  devised 
which,  in  a  measure,  remedy  this  defect.  After  it  is  once  dried,  flesh 
may  be  preserved  in  this  form  almost  indefinitely.  The  drying  of 
flesh  is  a  process  which  hardly  concerns  agriculture  in  this 
country. 

The  same  end  is  very  commonly  reached  on  the  farm  by  artificial 
drying,  accompanied  by  salting  and  smoking.  In  the  preparation  of 
smoked  hams,  bacon,  or  other  flesh,  bacterial  growth  is  prevented, 
partly  by  the  drying  and  partly  by  the  actual  germicidal  action  of  the 
smoke.  When  the  smoke  is  produced  from  certain  woods — beech 
wood  is  especially  favorable — various  volatile  products  arise,  such  as 
phenol  and  creasote,  and  these  act  as  germicides.  The  bacteria  on 
the  surface  of  the  meat  are  destroyed,  and  the  surface  is  dried  and 
affected  by  the  volatile  products  in  such  a  way  that  bacteria  will  not 
readily  start  to  grow  upon  the  flesh.  Smoked  meats  are  thus  pre- 
served, in  part  by  the  drying,  and  in  part  by  the  action  of  the  smoke. 


PRESERVATION    BY    DRYING.  239 

It  is  well  to  remember  that  drying  and  smoking  do  not  kill  the  animal 
parasites  that  may  be  in  the  meat,  like  trichina  or  tape- worms. 

Fruit. — The  drying  of  apples,  squashes,  pumpkins,  and  other 
vegetables  is  a  common  process  of  farm  life.  In  warmer  regions  of 
the  earth  the  sun's  rays  are  sufficient  to  dry  many  fruits  for  preserva- 
tion. Raisins  and  figs  are  thus  prepared.  In  colder  regions  arti- 
ficial heat  must  be  employed.  By  the  use  of  artificial  heat  it  has 
been  found  possible  to  preserve,  by  drying,  a  large  number  of  fruits. 
Pears,  prunes,  plums,  raspberries,  blackberries,  blueberries,  and  straw- 
berries represent  some  of  the  farm  products  which  readily  yield  to 
this  method  of  treatment.  In  fruit  prepared  in  this  way  the  water 
is  not  all  removed,  sometimes  as  much  as  30  per  cent,  being  left. 
In  most  cases  there  is  considerable  sugar  in  the  dried  product  which 
aids  in  the  preservation.  In  pears  there  is  some  30  per  cent,  of 
sugar,  while  in  raisins  there  is  about  60  per  cent.  It  must  always 
be  remembered  that  drying  does  not  destroy  the  bacteria,  but  only 
checks  their  growth,  and  if  the  fruit  has  been  exposed  to  a  possible 
contamination  of  pathogenic  bacteria,  the  drying  does  not  remove 
the  danger.  This  method  of  preserving  fruits  naturally  affects  their 
flavor  and  is  frequently  quite  unsatisfactory  for  this  reason,  although 
it  does  not  materially  affect  their  nutritive  value.  In  recent  years 
hydraulic  pressure  has  been  used  to  extract  the  water  with  results, 
on  the  whole,  superior  to  the  extraction  by  simple  drying. 

Hay. — One  of  the  most  important  applications  of  the  drying 
process  is  in  the  preparation  of  hay.  The  fresh  grass  contains 
so  much  moisture  that  it  could  not  be  preserved  in  masses  without 
undergoing  extensive  decomposition,  and  to  obviate  this  the  farmer 
resorts  to  the  simple  plan  of  drying  out  some  of  the  water.  But 
thisnphenomenon  of  drying  is  not  always  as  simple  as  it  looks,  and 
sometimes  a  fermentation  is  certainly  involved.  Where  the  climate 
is  moderately  dry  and  the  sun  hot,  the  simple  method  of  exposing 
th.^.  grass  to  the  sun  ^or  a  few  hours  is.  most  widely  adopted.  But 
such  a,  method  is  not  possible  in  regions  where  there  is  likely  to 
b^ia  grfjat  deal  of  rain. 

:^:Cmmg  of  Hay.  by  Self -fermentation. — In  countries  where  rains 
are  frequent  and  sunshine  rare,  the  sun's  rays  cannot  be  depended 


240         THE  PRESERVATION  OF  FOOD  PRODUCTS. 

upon  for  the  curing  of  hay,  so  a  different  principle  is  used.  If  the 
moist  grass  is  heaped  in  piles  and  allowed  to  stand  for  a  few  days, 
there  appears  a  marked  rise  in  temperature.  This  continues  rapidly, 
the  rate  and  the  temperature  reached  depending  upon  the  conditions: 
the  denser  the  packing,  the  higher  the  temperature.  Commonly 
the  rise  is  not  above  160°,  although  under  some  conditions  it  goes 
above  this,  even  to  the  point  of  spontaneous  combustion.  This  latter 
phenomenon  is  of  rare  occurrence,  however,  although  probably  it 
is  the  cause  of  the  spontaneous  combustion  that  occurs  occasionally 
in  a  barn  when  the  hay  is  packed  away  in  too  moist  a  condition. 

The  cause  of  such  self-heating  has  not  been  definitely  settled. 
It  is  evident  that  the  phenomenon  has  a  decided  resemblance 
to  the  fermentation  of  tobacco  and  also  to  that  of  silage. 
Three  possible  causes  may  be  concerned:  i.  The  respiratory 
changes  in  the  still  living  cells  of  the  grass.  2.  The  action  of 
enzymes  from  the  grass.  3.  The  growth  of  microorganisms. 
Experimental  tests  have  not  yet  settled  positively  the  relation 
between  these  possibilities.  Sterilized  hay  will  not  undergo  this 
heating,  while  the  same  sterilized  hay,  if  inoculated  with  certain 
species  of  microorganisms  (Oidium),  will  show  a  rise  in  temperature 
apparently  identical  with  the  self-heating.  This  would  clearly 
indicate  that  microorganisms  may  be  prominently  concerned 
in  the  process.  But  the  sterilizing  kills  the  plant  cells  so  that  the 
respiratory  changes  are  stopped,  and  also  destroys  most  of  the 
enzymes  present.  Hence  the  fact  that  sterilized  hay  will  not  thus 
heat  is  no  proof  that  microorganisms  alone  are  concerned  in  the 
phenomenon.  Further,  it  is  extremely  improbable  that  bacteria 
or  molds  could  develop  heat  sufficient  to  kill  themselves,  still  less 
sufficient  to  cause  a  spontaneous  combustion.  No  experiment  with 
organisms  has  given  a  heat  higher  than  160°  as  the  result  of  their 
action.  Hence,  the  extreme  heat  must  be  due  to  other  causes. 
Probably  this  fermentation,  like  many  another,  is  of  a  mixed  nature. 
T^he  moist  grass  still  contains  some  living  cells  that  for  a  time 
remain  alive  and  carry  on  respiratory  processes;  the  enzymes  in 
the  grass  probably  also  start  some  chemical  action  and,  lastly,  the 
microorganisms  on  the  grass,  by  their  growth,  add  to  the  fermenta- 


PRESERVATION    BY   DRYING.  24I 

tive  changes  going  on  in  the  hay.  As  a  result  of  all  three,  the 
temperature  rises. 

This  self-heating  is  utilized  in  some  countries  to  cure  the  hay. 
The  grass  is  built  up  into  a  stack  or  rick  13  to  16  feet  high,  and 
16  to  24  feet  in  diameter.  It  is  well  trodden  down,  but  not  firmly 
packed,  and  the  whole  stack  is  thatched  so  as  to  shed  the  rain. 
In  such  ricks  a  spontaneous  fermentation  sets  up  and  the  mass 
becomes  heated.  The  temperature  frequently  rises  as  high  as 
160°  F.,  but  not  much  higher,  and  there  is  no  danger  of  spontaneous 
combustion.  The  rick  is  not  opened,  but  the  hay  remains  in  the 
mass  until  the  farmer  wishes  to  use  it.  It  is  immaterial  whether 
the  hay  is  rained  on  or  not,  and  this  makes  the  process  especially 
adapted  to  rainy  districts. 

The  fermentation  which  takes  place  in  these  ricks  produces  a 
great  change  in  the  nature  of  the  product.  It  becomes  a  firm, 
dry  mass,  of  a  pale  or  dark  brown  color  or,  if  the  heating  is  too  great, 
it  may  be  almost  black.  It  has  developed  at  the  same  time  an 
aromatic  odor  which  resembles  freshly  baked  bread.  There 
develops  also  a  large  amount  of  lactic  and  butyric  acids,  the  amount 
of  lactic  acid  being  as  high  as  7  per  cent,  and  the  butyric  acid  over  2 
per  cent.  These  acids  are  derived  chiefly  from  the  carbohydrates, 
as  is  shown  by  the  great  reduction  in  the  amount  of  these  bodies  in 
the  drying  hay.  A  considerable  part  of  the  nitrogen  material  is  also 
lost,  the  total  loss  in  the  hay  being  about  14  per  cent.  This  loss  of 
material  is  one  of  the  objections  to  this  method  of  curing  hay. 
It  is  known  as  brown  hay. 

Sometimes  a  slightly  different  method  is  used.  The  freshly 
mown  grass  is  piled  in  heaps  from  10  to  13  feet  high,  the  mass  being 
trodden  down  as  tightly  as  possible  to  prevent  the  admission  of 
air.  The  temperature  in  these  heaps  rises  rapidly,  and  is  tested 
by  a  thermometer.  When  it  rises  to  about  158°  F.,  which  occurs 
generally  in  from  48  to  60  hours,  the  heaps  are  opened  and  spread 
out  in  thin  layers  to  the  air.  The  heat  in  the  hay  now  rapidly  dries 
the  product  and,  with  a  single  turning,  it  is  ready  for  storing.  Hay 
thus  prepared  is  called  burnt  hay,  and  develops  an  aromatic  odor 
which  ordinary  sun-dried  hay  does  not  possess. 


242  THE    PRESERVATION    OF    FOOD    PRODUCTS. 

Lastly,  it  should  be  noticed  that  ordinary  sun-dried  hay  will 
sometimes,  especially  if  stored  in  too  moist  a  condition,  undergo 
a  similar  heating  in  the  mow.  The  hay  may  be  considerably 
injured  by  such  heating,  so  that  it  will  lose  some  of  its  nutriment. 
Sometimes  the  heat  is  sufficient  to  cause  an  actual  igAition  of  the 
hay. 

Certain  phenomena  sometimes  seen  in  cotton  are  clearly  closely 
akin  to  the  fermentation  just  described,  for  cotton  may  undergo  a 
spontaneous  heating  sufficient  to  render  it  in  danger  of  combustion, 
and  this  must  be  due  to  processes  similar  to  those  just  mentioned. 
The  same  thing  is  true  of  hops  which  occasionally  develop  a  like 
spontaneous  heating  during  the  curing. 

PRESERVATION  BY  COLD. 

All  the  common  species  of  bacteria  grow  more  slowly  as  the 
temperature  is  lowered,  and  cease  growing  entirely  when  it  reaches 
freezing.  The  nearer  to  freezing  a  fermentable  substance  is  kept, 
the  greater  the  delay  of  the  bacterial  growth.  In  the  large  cold- 
storage  houses  the  food  which  is  to  be  preserved  may  be  cooled 
to  a  temperature  below  freezing,  and  is,  consequently,  actually 
frozen.  At  this  temperature  the  bacteria  never  act  and  the  material 
may  be  kept  indefinitely,  although  it  is  claimed  that  some  molds 
can  grow  at  temperatures  below  freezing.  It  must  be  remembered, 
however,  that  the  low  temperatures  do  not  kill  the  bacteria,  but 
simply  delay  their  action,  and  as  soon  as  such  food  products  are 
warmed,  the  bacteria  immediately  begin  to  grow.  Indeed,  food 
spoils  very  rapidly  when  taken  from  cold  storage. 

Where  such  low  temperatures  are  not  feasible,  a  moderate  de- 
gree of  cold  may  check  bacterial  growth  and  delay  putrefaction. 
An  ice  chest  usually  maintains  a  temperature  below  50°,  and  this  is 
very  efficient  in  helping  preserve  food.  A  cool  cellar  answers  the 
same  purpose,  as  well  as  the  common  plan  of  placing  milk  and  other 
foods  in  cold  water  to  delay  the  spoiling.  Fruits  are  particularly 
benefited  by  these  cool  dry  temperatures,  and  a  cellar  which  has  a 
fairly  uniform  and  low  temperature  is  of  great  value  on  the  farm  in 


PRESERVATION    BY    THE    USE    OF    CHEMICALS.  243 

helping  to  preserve  fruits  and  vegetables  that  would  otherwise  soon 
spoil.  The  value  of  ice  as  an  aid  in  keeping  all  sorts  of  perishable 
material  is  too  fully  understood  to  require  further  notice. 

PRESERVATION  BY  THE  USE  OF  CHEMICALS. 

Many  chemical  substances  are  destructive  to  bacteria,  and  foods 
may  frequently  be  protected  from  bacterial  action  by  the  addition  of 
small  quantities  of  some  material,  harmless  in  itself,  yet  having 
a  checking  action  upon  bacteria.  Such  agents  are  called  preserva- 
tives. If  they  are  to  be  used  in  the  preservation  of  food  products,  it 
is  of  course  necessary  that  they  should  not  be  deleterious  to  health 
and  also  that  they  should  not  impart  disagreeable  flavors  to  the  food. 
The  number  of  substances  that  can  be  used  without  hesitation  is  not 
very  great. 

The  more  powerful  antiseptics,  like  carbolic  acid  and  corrosive 
sublimate,  are,  of  course,  out  of  the  question.  Certain  milder  ones, 
borax  J  boracic  acid,  salicylic  acid,  formalin  and  benzoic  acid,  are  more 
or  less  extensively  used.  There  are  on  the  market  various  com- 
mercial preservatives,  Preservaline,  Anti-fermentine,  Freezine,  etc. 
These  several  articles  have  different  compositions,  but  all  are  wholly 
or  in  part  made  up  of  the  substances  named,  most  of  them  being 
either  borax  or  formalin.  They  are  undoubtedly  efficient  in  pre- 
venting putrefaction  and  decay,  for  they  are  antiseptics,  and  if  used 
in  sufficient  quantity  will  stop  bacteria  growth.  They  have  been 
widely  used  in  meats  and  in  milk. 

But  the  question  arises  whether  they  are  not  injurious  to  health. 
Each  is  injurious  to  man  if  taken  in  sufficient  quantity.  Are  they, 
then,  objectionable  in  the  small  quantities  used  in  preserving  food  ? 
This  question  has  led  to  much  experimenting,  especially  by  the 
U.  S.  Department  of  Agriculture.  The  general  result  of  these  exten- 
sive experiments  is  to  show  that  these  preservatives,  when  used  in 
small  amounts  day  after  day,  are  injurious,  although  this  conclusion 
is  still  disputed.  Hence,  the  general  conclusion  is  that  the  pre- 
servatives in  question  are  to  be  condemned.  They  are  certainly 
illegitimate,  and,  since  the  same  results  can  be  reached  in  another 


244         THE  PRESERVATION  OF  FOOD  PRODUCTS. 

way,  there  is  no  excuse  for  their  use  in  any  ordinary  food  products. 
They  are  especially  to  be  condemned  in  milk. 

NON-POISONOUS   PRESERVATIVES. 

Salt. — Salt  is  not  an  antiseptic  in  any  proper  sense  and  it  does 
not  destroy  bacteria.  But  it  may  be  a  preservative  and  when  much 
of  it  is  present  in  a  solution,  it  has  a  decidedly  repressing  action  upon 
bacterial  growth,  and  may  stop  the  ordinary  putrefactive  changes. 
When  used  in  the  preservation  of  butter  and  fish,  it  also  has  the 
advantage  of  imparting  a  relish  to  the  product.  It  is  in  general  use 
for  the  preservation  of  flesh  of  various  kinds.  Flesh  which  is  to  be 
smoked  is  commonly  first  salted,  the  salt  adding  to  the  efficacy  of 
this  method  of  preservation.  Salt  pork  is  pork  preserved  in  strong 
salt  brine,  and  corned  heef  and  corned  bacon  are  preserved  in  much 
the  same  way.  Ham  is  partly  preserved  by  salt,  and  fish  wholly 
so.  Butter  and  cheese  both  have  their  keeping  qualities  increased  by 
salt.  But  salt  used  in  this  way  does  not  kill  the  bacteria,  and  any 
flesh  that  contains  injurious  organisms,  bacteria  or  others,  is  not 
rendered  wholesome  by  salting. 

Sugar. — A  moderate  amount  of  sugar  checks  most  bacterial 
growth,  and  a  large  amount  even  stops  yeast  growth.  Sugar,  there- 
fore, is  widely  used  as  a  preservative.  Since  it  is  in  itself  a  good 
food  there  can  be  no  objection  to  its  use  as  a  preservative,  although 
it  always  changes  the  taste  and  nature  of  the  product.  Condensed 
milk  may  contain  30  to  40  per  cent,  of  sugar.  Jellies,  preserves,  jams, 
marmalades  are  all  fruits  prepared  in  various  ways  and  mixed  with 
more  or  less  sugar  as  a  preservative.  Raisins,  figs,  and  prunes  are 
whole  fruits  partly  dried  and  preserved  by  the  drying  and  the 
large  percentage  of  sugar  contained  in  them.  There  are  practical 
difficulties  in  the  way  of  using  sugar  with  some  foods,  but  with 
others  it  has  its  value. 

Vinegar. — Acetic  acid  is  another  legitimate  food  preservative, 
and  is  extensively  used  in  the  manufacture  of  pickles.  The  acid 
gives  a  sharp  taste  to  the  pickles  and  also  largely  prevents  the  growth 
of  the  common  putrefying  organisms.     The  vinegar  is  frequently 


PRESERVATION   BY    CANNING.  245 

mixed  with  spices,  both  for  the  purpose  of  adding  flavor  and  of  aid- 
ing in  the  preservation.  Vinegar  pickles  will  not  keep  indefinitely,  for 
after  a  time  a  scum  grows  over  the  surface.  This  is  made  of  micro- 
organisms which  gradually  weaken  the  strength  of  the  vinegar  until 
the  final  decay  of  the  pickles  is  only  a  matter  of  time,  unless  precau- 
tion is  taken  to  prevent  the  deterioration  of  the  vinegar. 

Spices. — Many  common  household  spices  are  more  or  less 
efficient  as  antiseptics  and  tend  to  delay  putrefaction.  In  some 
kinds  of  pickles  spices  are  used,  and  mince  meat,  sausages,  and 
highly  spiced  fruit  cakes  are  preserved  chiefly  by  the  spices  they 
contain. 

PRESERVATION  BY  CANNING. 

One  of  the  most  important  methods  of  preserving  perishable 
food  products  was  invented  a  century  ago,  long  before  the  signifi- 
cance of  bacteria  was  known  and  long  before  the  meaning  of  the 
process  was  understood.  It  was  invented  by  Appert,  a  Paris  con- 
fectioner in  1810,  and  consisted  first  in  boiling  the  material  to  be 
preserved  and  subsequently  sealing  it  hermetically.  It  was  at  first 
supposed  that  the  significance  of  the  sealing  was  to  prevent  the  access 
of  air,  but  it  is  now  known  that  its  purpose  is  simply  to  prevent  the 
entrance  of  bacteria;  for  if  these  can  be  kept  out,  the  presence  of  air 
does  not  interfere  with  the  preservation.  It  is  interesting  to  note 
that  this  method  of  preservation  of  food  products  was  invented  and 
put  to  a  wide  practical  use  while  scientists  were  disputing  and  labori- 
ously experimenting  over  the  problem  of  spontaneous  generation. 
The  experiments  by  which  scientists  tried  to  settle  this  question  con- 
sisted in  exactly  the  same  devices  as  just  mentioned,  viz.,  the  heating 
of  various  organic  materials  to  a  high  temperature  in  order  to  kill  all 
living  organisms  and  then,  after  hermetically  sealing,  watching  to 
see  if  life  developed  in  the  sterilizing  mass.  While  the  scientists 
were  disputing  as  to  the  results,  the  method  of  canning  was  put  into 
practical  use,  and  every  can  of  preserved  fruit  was  evidence  against 
spontaneous  generation. 

The  general  method  adopted  in  canning  is  well  known.     Micro- 


246  THE   PRESERVATION    OF    FOOD    PRODUCTS. 

organisms  are  so  abundant  everywhere  that  every  bit  of  food  is 
certain  to  be  infested  with  them.  The  first  step  taken  must  be 
the  killing  of  all  the  organisms  that  may  be  adhering  to  the  food  to 
be  preserved.  This  is  done  by  heat,  the  material  being  commonly 
placed  first  in  the  receptacles  in  which  it  is  to  be  finally  sealed.  The 
amount  of  heat  necessary  varies  much  with  the  nature  of  the 
material.  If  it  is  of  a  proteid  nature,  like  meat,  beans,  peas,  or 
corn,  it  is  likely  to  contain  spore-forming  bacteria,  and  will  require 
high  and  prolonged  heating.  This  is  especially  true  of  corn, 
peas,  and  beans,  since  these  materials  contain  such  resisting  spores 
that  it  was  once  thought  that  they  could  not  be  successfully  canned. 
But  modern  methods  of  applying  heat  above  the  boiling  tempera- 
ture have  made  it  possible  to  sterilize  thoroughly  even  these  resisting 
substances,  and  their  canning  is  now  perfectly  feasible.  To-day 
temperatures  of  230°  to  250°  are  commonly  used  for  such  materials. 
In  canning  fruit,  as  a  rule,  such  high  heat  is  not  needed,  since 
fruits  are  more  often  spoiled  by  yeasts  and  molds  than  by  spore- 
bearing  bacteria,  and  these  organisms  are  easily  killed  by  simple 
boiling,  or  by  even  less  heat. 

The  second  step  is  the  sealing.  This  consists  simply  in  closing 
the  vessel  containing  the  sterilized  mass  in  such  a  way  as  absolutely 
to  exclude  the  air  and  with  it  all  microorganisms.  It  is  sometimes 
done  in  tin  cans,  when  a  small  hole  is  left  in  each  can  so  that  the 
air  and  steam  may  escape  during  the  sterilizing,  after  which  process 
the  hole  is  sealed  by  a  drop  of  solder.  Sometimes  glass  jars  are 
used,  and  these  are  sealed  by  covers  pressed  forcibly  down  upon 
a  soft  rubber  ring.  The  principle  is  the  same  in  either  case.  The 
sterilized  food  thus  removed  from  any  possible  means  of  contamina- 
tion will  keep  indefinitely. 

The  development  of  the  canning  industry  does  not  belong  to 
our  immediate  subject,  but  certain  facts  connected  with  the  matter 
have  produced  great  changes  in  the  possibilities  of  agriculture. 
It  has  made  possible  the  utilization  of  a  great  quantity  of  food 
products  which  otherwise  could  not  be  used.  Certain  of  our  fruits 
are  extremely  palatable  but  very  perishable,  and  if  it  were  necessary 
to  use  them  fresh  or  in  a  dry  condition,  only  comparatively  small 


■    PRESERVATION    BY    CANNING.  247 

quantities  could  be  raised.  For  example:  before  the  beginning  of 
tomato  canning,  only  a  very  small  crop  could  be  utilized;  but  the 
opening  of  this  canning  industry  has  entirely  changed  the  conditions, 
and  now  great  tracts  of  land  can  be  devoted  to  raising  this  delicacy, 
thus  opening  to  the  farmer  an  entirely  new  outlet  for  his  crop.  The 
same  is  true  of  many  another  farm  product.  It  is  no  longer  neces- 
sary for  the  farmer  to  depend  upon  his  own  market,  but,  by  the  aid 
of  canning,  his  market  may  be  the  world,  open  to  him  the  whole 
twelve  months  of  the  year.  The  canning  industry  makes  it  possible 
for  the  farmer  to  become  a  specialist,  where  it  was  impossible  a 
few  years  ago.     He  may  raise  green  corn,  or  tomatoes,  or  straw- 

A/       /      O.°°'o°» 


i» 


.-•-^ 


Fig,  50. — Three  species  of  bacteria  causing  the  spoiling  of  canned  corn 
{Prescott  and  Underwood). 


berries  as  abundantly  as  he  pleases,  and  whatever  he  cannot  find 
an  immediate  market  for  may  be  preserved  for  a  later  season  by  the 
process  of  canning.  It  is  well  for  the  agriculturist  to  learn  that 
in  farming,  as  in  all  other  industries,  it  is  the  specialist  who  succeeds, 
and  that  the  proper  utilization  of  the  process  of  canning  is  one  of 
the  means  of  making  a  special  product  upon  a  farm  yield  proper 
returns.  Canning  makes  possible  an  intensive  farming,  undreamed 
of  a  few  years  ago. 

The  present  condition  of  the  canning  industry  has  been  reached 
only  after  years  of  experience,  accompanied  with  many  failures  and 
losses.  Whole  shipments  have  sometimes  been  ruined  by  "  swelling," 
which  means  that  the  cans  swell  out  from  the  force  of  the  putrefying 
gases  forming  within.     The  failure  to  appreciate  the  difl&culty  of 


248  THE    PRESERVATION    OF    FOOD    PRODUCTS. 

killing  bacteria  spores  has  caused  great  losses  of  canned  corn,  peas, 
and  beans,  as  well  as  tomatoes.  It  must  be  recognized  that,  for 
successful  canning,  every  spore  must  be  killed;  for  if  a  single  one 
be  left  alive  in  the  middle  of  the  can,  the  product  is  sure  to  spoil 
(Fig.  50).  The  slowness  with  which  heat  will  pass  to  the  middle  of 
the  can  was  not  recognized  until  many  losses  had  resulted  from 
insufficient  heating.  But  the  failures  proved  instructive,  and  after 
bacteriologists  studied  the  different  problems  presented  by  the 
attempts  to  can  food  stuffs,  the  questions  were  answered  one  by 
one,  and  successful  rules  were  devised  for  the  canning  of  any  food 
products  subject  to  such  methods.  To-day  failures  are  rare  and 
are  all  attributable  to  carelessness  in  the  process.  So  thoroughly 
has  this  subject  been  mastered  that  to-day  any  food  that  can  stand 
heat  may  be  perfectly  preserved  by  boiling  and  canning.  Of  course 
the  flavors  are  commonly  changed  by  the  process,  and  few  of  them 
appear  like  the  fresh  material.  But  usually  the  flavors  are  less 
changed  than  by  any  other  method  of  preservation,  and  canned 
goods  are  vastly  superior  to  the  dried  foods  with  which  our  grand- 
fathers were  forced  to  be  content  in  winter.  Canning  is  especially 
adapted  to  foods  containing  a  good  deal  of  water,  and  hence  is 
of  especial  use  among  foods  that  cannot  be  well  preserved  by  drying. 
Fruits  are  well  adapted  to  canning,  but  ill  adapted  to  drying,  and 
are  ruined  by  salting. 

BACTERIA  IN  EGGS. 

The  presence  of  bacteria  in  eggs  results  in  trouble  experienced 
by  every  farmer,  and  one  which  it  seems  impossible  to  avoid. 
It  might  be  supposed  that  eggs,  when  freshly  laid,  would  be  free 
from  bacteria  and  hence  not  liable  to  decay.  But  this  is  certainly 
not  the  case.  Bacteria  are  known  to  enter  the  oviduct  and  contami- 
nate the  mass  of  the  egg  even  before  its  shell  is  deposited.  Hence 
when  the  egg  is  laid  it  will  commonly  contain  bacteria  in  greater  or 
less  numbers.  These  bacteria  can  obtain  plenty  of  oxygen  from 
the  air  that  enters  through  the  porous  shell,  and  are  thus  able  to 
grow  readily  within  the  egg,  where  they  soon  cause  its  decay.     A 


BACTERIA    IN    EGGS.  249 

bacteriological  study  of  eggs  has  shown  quite  a  number  of  different 
bacteria  in  perfectly  whole  eggs,  freshly  laid,  and  there  seems 
to  be  no  possible  means  of  avoiding  them  completely.  Even 
after  the  shell  is  deposited  and  the  egg  laid,  bacteria  are  capable 
of  entering  it.  The  shell  is  somewhat  porous  and  it  has  been 
proved  by  experiment  that  bacteria  can  pass  through  the  pores. 
In  short,  the  egg  must  be  looked  upon  as  a  highly  nutritious  food 
product,  in  most  cases  already  inoculated  with  bacteria,  and  a 
body  which  is  practically  suie  to  undergo  decay  in  the  course  of 
time.  There  are  fewer  bacteria  in  winter  eggs  than  in  those  laid  in 
summer,  and  the  cleaner  the  nest  the  less  the  bacterial  contam- 
ination. 

It  is,  however,  possible,  by  certain  devices,  to  delay  or  prevent  the 
growth  of  bacteria  in  the  egg.  The  bacteria  found  in  eggs  do  not  de- 
velop at  low  temperatures  and  the  eggs  may  be  kept  almost  indefi- 
nitely at  a  temperature  of  34°.  But  since  cold  storage  is  not  always 
available,  other  methods  must  usually  be  adopted.  One  of  the 
simplest  and  best  is  by  the  use  of  water  glass,  a  material  made  of 
sodium  and  potassium  silicate.  This  may  be  purchased  cheaply 
in  the  form  of  a  thick  syrup.  It  is  then  mixed  with  nine  parts 
of  water  and  placed  in  clean  stone  jars.  The  eggs  are  placed  in 
the  mixture  and  the  whole  set  aside  in  a  cool  place.  If  the  tem- 
perature is  not  allowed  to  rise  above  60°  the  eggs  may  be  kept  from 
decay  for  a  long  time  by  this  method,  many  weeks  and  even  months 
elapsing  before  they  will  decay.  In  preserving  eggs  in  this  way  it  is 
important  to  know  that  April  eggs  will  keep  better  than  May  eggs 
and  these  better  than  June  eggs,  while  eggs  laid  in  the  hot  months 
of  the  summer  are  less  easily  preserved,  a  fact  probably  due  to  a 
greater  bacterial  contamination  during  the  warmer  months.  From 
this  it  will  follow  that  the  June  storage  eggs  should  be  used  first 
and  the  April  preserved  eggs  last. 

Eggs  thus  preserved  will  keep  from  decay,  but  they  will  lose  their 
fresh  taste.  Indeed,  this  fresh  flavor  disappears  in  a  very  few  days 
and  there  is  no  way  known  by  which  it  can  be  retained  for  very  long. 
But  after  the  fresh  taste  is  gone  the  eggs  will  remain  without  further 
change  for  a  long  time  and  be  usable. 


250  THE    PRESERVATION    OF    FOOD    PRODUCTS. 

BACTERIA  IN  THE   SUGAR  INDUSTRY. 

A  brief  mention  should  be  made  of  the  relation  of  bacteria  to  the 
sugar  industry,  which  is  an  important  phase  of  agriculture.  The 
relation  of  microorganisms  to  this  industry  is,  according  to  our 
present  knowledge,  only  one  of  injury.  After  the  product  has  been 
harvested,  bacteria  may  produce  subsequent  injury  within  it,  giving 
rise  to  well-known  troubles.  One  source  of  trouble  experienced  in 
sugar-making  consists  in  the  appearance,  at  various  stages  of  manu- 
facture, of  jelly-like  masses  which  may  become  very  abundant  and 
troublesome.  This  has  long  been  known  and  has  been  studied  by 
bacteriologists  for  many  years,  with  the  result  of  proving  that  it  is 
caused  by  the  appearance  and  development  of  certain  species  of  bac- 
teria. ■  Several  species  are  known  and  have  been  carefully  studied, 
al  1  of  which  have  the  power  of  producing  a  slimy  section  which  gives 
rise  to  the  jelly-like  masses  in  the  sugar  product.  The  slimy  secre- 
tion appears  to  be  developed  from  the  sugar,  a  conclusion  proved 
by  the  fact  that  the  same  microorganism,  when  growing  out  of  con- 
tact with  sugar,  develops  no  slime.  A  second  trouble  is  in  the  loss 
of  sugar  by  inversion.  This  occurs  in  unrefined  sugar  during  stor- 
age or  transportation,  and  is  due  to  a  bacterium  that  has  been 
isolated  and  tested. 


PART  V. 

CHAPTER  XVII. 
PARASITIC  BACTERIA. 

RESISTANCE  AGAINST  PARASITIC  BACTERIA. 

We  have  learned  that  microorganisms  may  be  both  useful  and 
harmful.  If  they  grow  where  they  are  wanted,  they  are  useful; 
but  if  they  grow  where  they  are  not  wanted,  they  produce  many 
undesirable  effects.  They  spoil  foods  by  causing  their  putrefaction, 
they  destroy  vinegar  by  consuming  the  acetic  acid.  In  wines  the 
growth  of  mischievous  microorganisms  causes  a  variety  of  bad 
results  that  are  sometimes  spoken  of  as  "diseases  of  wine,"  and  we 
also  hear  of  "  diseases  of  beer."  But  there  is  no  good  reason  for  the 
use  of  the  term  here  any  more  than  in  speaking  of  the  diseases  of 
butter  and  cheese,  when  unusual  bacteria  cause  them  to  ripen 
abnormally. 

In  most  of  the  examples  thus  far  studied  the  material  upon  which 
the  microorganisms  grow  has  been  supposed  to  be  lifeless,  the 
bacteria  existing  as  saprophytes.  There  remains  the  study  of  these 
organisms  when  growing  upon  the  living  tissues  of  animals,  thus 
living  the  life  of  parasites.  In  the  latter  case  they  may  do  injury  to 
the  animal  or  plant  upon  which  they  live,  thus  becoming  pathogenic, 
or  disease  germs. 

HOW  MICROORGANISMS  PRODUCE  DISEASE. 

When  they  multiply  inside  the  body,  microorganisms  show  very 
different  habits.  Sometimes  they  become  distributed  over  the 
whole  body,  located  at  no  particular  spot  (blood  poisoning),  while  in 
other  cases  they  may  be  definitely  localized  at  some   one   place 

251 


252  PARASITIC   BACTERIA. 

{diphtheria).  Between  such  extremes  there  are  many  intermediate 
types.  Whenever  the  microorganisms  muhiply  in  the  body  they 
produce  chemical  changes,  just  as  they  do  elsewhere.  New  chemi- 
cal bodies  are  secreted  by  them  and  among  these,  in  the  case  of 
disease  germs,  there  are  some  that  are  poisonous  in  their  nature. 
Such  substances  are  called  toxins.  Wherever  they  are  produced 
they  are  liable  to  be  absorbed  by  the  blood,  and  the  body  may  thus 
be  directly  poisoned  by  them.  If  the  bacteria  are  in  the  blood  it- 
self, this  poisoning  is  easy  to  understand,  but  localized  diseases  are 
similarly  explained.  Diphtheria,  for  example,  is  produced  by 
bacteria  growing  on  the  inside  surface  in  the  throat.  The  bacteria 
themselves  do  not  enter  the  body,  but,  growing  in  the  throat,  they 
develop  very  powerful  toxins,  and  these  are  absorbed  into  the  blood, 
producing  a  general  poisoning  of  the  whole  body.  All  disease 
germs  produce  poisonous  materials  which  are  absorbed  by  the 
body,  and  these  cause  the  direct  injury  characteristic  of  the  various 
diseases. 

RESISTANCE  AGAINST  MICROORGANISMS. 

A  very  large  majority  of  microorganisms  are  quite  unable  to  live 
within  the  bodies  of  living  animals  or  plants,  and  therefore  are  not 
parasitic.  If  common  putrefactive  bacteria  be  inoculated  into  the 
blood  of  a  living  cow  or  into  her  flesh,  they  will  speedily  die  without 
multiplying,  disappearing  in  a  very  short  time.  If  these  same 
bacteria  are  inoculated  into  the  same  animal  after  it  is  dead,  they 
will  grow  with  rapidity,  quickly  causing  the  flesh  to  putrefy.  Why 
is  there  this  difference  ?  The  complete  answer  to  this  question  is  one 
for  which  bacteriologists  have  long  been  searching,  but  have  as 
yet  only  partly  found.  A  partial  answer  is  that  the  living  tissues 
contain  substances  that  are  injurious  to  the  bacteria.  What  these 
substances  are,  how  they  act,  why  they  disappear  after  death,  and 
numerous  other  questions  concerning  them  are  among  the  most 
important  of  the  problems  before  bacteriologists  to-day.  With  these 
complicated  questions,  however,  we  are  not  concerned  in  this  work. 

It  is  evident  enough  that  some  kinds  of  microorganisms  can 


RESISTANCE  AGAINST   MICROORGANISMS.  253 

overcome  this  resistance,  otherwise  there  would  be  no  parasites. 
These,  capable  of  living  and  multiplying  in  the  body,  may  produce 
injury  and  are  the  disease  germs.  Fortunately,  the  number  that 
can  thus  live  is  small.  Many  hundred  kinds  of  bacteria  have 
been  discovered,  carefully  studied,  and  described  in  bacteriological 
literature.  We  have  no  idea  how  many  varieties  exist  in  nature, 
but  there  are  certainly  hundreds,  and  perhaps  thousands.  Of 
these  only  little  more  than  a  score  are  known  that  can  produce  disease 
in  man  and  animals,  and  a  somewhat  larger  number  that  can  produce 
disease  in  plants.  A  few  yeasts  occasionally  produce  similar  troubles. 
Quite  a  large  number  of  molds,  as  parasites,  give  rise  to  disease  in 
plants,  and  a  very  few  cause  trouble  in  animals.  The  great  host 
of  bacteria  and  other  fungi  live  upon  dead  matter,  and  cannot 
live  as  parasites.  They  may  spoil  foods,  and  destroy  wines,  beer, 
butter,  cheese,  and  other  valuable  substances,  but  they  cannot 
produce  disease,  since  they  are  not  able  to  overcome  the  resistance 
offered  by  the  living  tissue. 

The  body  has  a  resisting  power  against  all  kinds  of  micro- 
organisms, disease  germs  as  well  as  the  non-parasitic  species, 
although,  in  the  case  of  the  former,  it  is  insufficient  to  prevent  their 
invasion.  Against  the  common  saprophytes  it  is  perfectly  efficient; 
against  some  parasitic  bacteria  it  is  moderately  efficient  and  will, 
in  many  cases,  prevent  the  development  of  the  disease,  even  after 
the  parasitic  bacteria  have  entered  (tuberculosis);  against  other 
bacteria  the  resisting  power  is  extremely  slight  (anthrax).  The 
resisting  power  varies  with  different  species  of  animals,  some 
having  the  power  of  absolutely  resisting  certain  bacteria,  when  we 
call  them  immune.  Man  is  immune  against  hog  cholera,  while 
the  hog  is  not.  It  also  varies  with  the  individual,  some  members 
of  a  species  having  the  resisting  power  highly  developed,  while 
others  yield  easily  to  invasion.  This  we  speak  of  as  individual 
resistance.  The  resistance  varies  also  with  the  vigor  of  the  germs. 
Some  epidemics  of  measles,  for  example,  are  mild  and  some  severe, 
and  a  person's  resistance  against  an  attack  is  partly  dependent  upon 
the  vigor  of  the  germs. 

Now,  this  resisting  power  is  clearly  located  in  the  living  cells  of 


254  PARASITIC   BACTERIA. 

the  body  and  is  dependent  upon  their  normal  functions.  It  is  only 
the  living  cell  which  can  resist  the  invasion  of  microorganisms, 
either  wholly  or  partially.  From  this  it  follows  that  the  resistance 
will  be  greatest  when  the  body  cells  are  in  the  highest  state  of 
physical  activity,  and  will  diminish  when  they  become  somewhat 
impaired  in  vitality.  Anything  which  tends  to  reduce  the  physical 
health  of  the  individual  tends  to  reduce  his  power  of  resistance. 
For  example:  sometimes  an  individual  shows  a  great  tendency 
to  develop  boils  or  abscesses,  and  but  little  power  of  resisting  them. 
We  say  his  "blood  is  in  a  bad  condition."  By  this  is  really  meant 
that  his  body  activities  are  so  repressed  that  he  is  unable  to  resist 
the  invasion  of  some  of  the  common  bacteria  which  are  present  on 
every  hand  and  which  an  individual  in  healthy  condition  easily 
repels.  If  his  physical  vigor  can  be  restored,  the  troubles  will 
disappear,  although  the  bacteria  which  produce  the  boils  and 
abscesses  are  just  as  abundant  around  him  as  before.  Physical 
vigor  is  the  best  protection  against  the  invasion  of  parasitic  bacteria, 
and  a  weakened  physical  condition  invites  attack. 

This  matter  is  emphasized  here  because  it  is  too  generally  lost 
sight  of  in  the  combat  against  infectious  diseases.  During  the  first 
years  of  the  study  of  bacteriology  there  was  a  very  general  tendency, 
in  the  attempt  to  avoid  diseases,  to  place  the  whole  emphasis 
upon  the  methods  of  avoiding  bacteria.  If  a  disease  is  caused  by  a 
bacterium,  what  more  natural  method  could  be  suggested  for 
avoiding  it  than  to  avoid  the  bacterium?  In  accordance  with  this 
idea  there  developed  a  long  series  of  rules  and  regulations  suggested 
by  bacteriologists  and  adopted  by  health  boards,  all  designed  for 
the  prevention  of  the  distribution  of  disease  germs.  This  is, 
indeed,  the  foundation  of  modern  sanitation. 

Increasing  of  Individual  Riesistance. — Recently  there  has 
been  a  manifest  reaction  against  this  one-sided  attitude.  While 
the  importance  of  preventing  the  distribution  of  bacteria  is  still 
acknowledged,  there  is  to-day  a  growing  recognition  of  another 
side  to  the  question.  The  strengthening  of  personal  vigor  is  of  no 
less  importance — many  believe  it  is  of  more  importance — than 
the  preventing  of  the  distribution  of  bacteria.     The  weakening  of 


RESISTANCE  AGAINST   MICROORGANISMS.  255 

personal  vigor  will  do  tnore  toward  increasing  germ  diseases  than 
a  relaxing  of  the  rules  which  try  to  prevent  the  distribution  of 
bacteria.  Personal  resistance  of  the  individual  will  enable  him  to 
repel  many  an  attack  of  disease  bacteria,  even  if  he  has  been  directly 
exposed  to  them,  while  a  weakened  resisting  power  may  result  in 
his  yielding  to  the  first  attack  of  an  invading  bacterium.  For  some 
of  the  less  violent  diseases  (tuberculosis)  this  is  much  more  emphatic- 
ally true  than  for  other  diseases  (anthrax).  Now,  it  is  not  possible 
to  hope  that  we  shall  ever  be  able  to  exterminate  all  pathogenic 
bacteria;  even  if  we  did,  other  forms  would  doubtless  take  their 
places.  Since  we  cannot  exterminate  them,  it  follows  that  all 
individuals  will,  at  some  time,  be  exposed  to  the  attacks  of  some 
of  the  disease  germs.  Manifestly,  then,  the  best  means  of  elevating 
the  healthfulness  of  the  race  is  to  raise  the  resisting  power,  at  the 
same  time  doing  our  utmost  to  destroy  pathogenic  bacteria. 

These  facts  are  equally  true,  whether  we  are  dealing  with  animals 
or  with  man.  It  is  of  more  importance  for  the  farmer  to  understand 
them  when  he  endeavors  to  make  a  fight  against  the  diseases 
of  domestic  animals  than  it  is  for  the  physician  or  the  veterinarian 
who  tries  to  cure  the  disease.  With  animals,  as  with  man,  the 
individual  resisting  power  is  variable.  When  a  lot  of  pigs  are 
attacked  by  that  very  fatal  disease,  hog  cholera,  some  of  them 
escape  with  no  signs  of  the  disease,  showing  a  superior  resisting 
power.  Undoubtedly  the  resisting  power  of  animals  is  due  to  a 
proper  physical  vigor,  little  understood,  but  plainly  dependent 
upon  proper  conditions  of  life.  Let  the  conditions  be  normal, 
and  the  animal  may  resist  the  attack  of  parasitic  bacteria;  but  let 
them  become  abnormal,  so  as  to  reduce  vitality,  and  the  animal 
is  much  more  likely  to  succumb. 

Tuberculosis,  for  example,  is  much  more  prevalent  among 
cattle  that  are  kept  stabled  most  of  the  time,  than  among  those  that 
spend  a  considerable  portion  of  the  time  in  the  open  air.  This  may 
be  due,  in  part,  to  the  fact  that  stabled  cattle  have  a  greater  chance 
of  acquiring  the  contagion,  since  they  are  kept  so  close  together. 
But  this  is  certainly  not  the  whole  reason.  Young  cattle  that  are 
kept  in  the  open  for  a  year  or  two  are  less  liable  to  take  the  disease 


256  PARASITIC   BACTERIA. 

than  those  kept  in  the  stable,  even  though  subsequently  they  are 
put  under  similar  conditions.  In  localities  where  the  animals  run 
out  of  doors  all  the  time  the  disease  is  rare.  The  more  closely  they 
are  housed  the  greater  the  tendency  to  this  disease,  and  it  is  practi- 
cally certain  that  this  greater  tendency  is  not  because  they  are  so 
much  more  likely  to  be  infected,  but  because  of  the  depressing  in- 
fluence which  such  a  restricted  life  has  upon  the  vitality  of  the 
animals,  reducing  their  resisting  powers.  It  is  also  a  general  be- 
lief that  highly  bred  cattle  have  a  greater  tendency  to  this  disease 
than  less  highly  bred  stock.  Stated  in  this  way  the  conception  may 
not  be  correct;  but  it  is  practically  certain  that  animals  which  have 
been  bred  for  the  purpose  of  producing  great  quantities  of  milk  are 
rather  more  likely  to  yield  to  the  disease  than  those  not  so  highly 
specialized.  Such  a  specialization  of  the  vitality  in  the  direction  of 
an  abnormally  high  action  of  the  milk  glands  cannot  fail  to  be  at  the 
expense  of  other  vital  functions.  These  breeds  have  been  developed 
in  one  direction  until  they  have  become  abnormal.  It  is  not  to  be 
wondered  at  if  such  an  abnormal  development  should  have  resulted 
in  the  reduction  of  their  general  vitality,  and  of  their  resisting  power 
against  disease.  It  is  the  active,  vigorous  cow,  which  produces, 
perhaps,  but  little  milk  and  is  not  carefully  housed  by  the  farmer 
that  has  the  power  of  resisting  disease.  In  short,  the  prevalence  and 
the  increase  of  some  of  the  diseases  of  domestic  animals  must  be 
attributed,  in  no  inconsiderable  measure,  to  the  introduction  into 
our  herds  of  conditions  of  life  which  lessen  their  resisting  power, 
and  not  wholly  to  the  increasing  chances  of  contagion  due  to  close 
contact  of  animal  with  animal.  That  the  latter  phenomenon  is 
also  a  factor  is,  of  course,  evident. 

The  conditions  of  life  among  domestic  animals  are,  to  a  very 
large  degree,  under  immediate  and  perfect  control.  We  can  regu- 
late the  amount  of  outdoor  life  they  have,  their  activity,  their  food, 
their  drink,  and  many  other  factors  upon  which  their  physical 
vigor  depends.  We  may  keep  the  cow  housed  so  that  she  has 
little  air;  we  may  give  her  highly  stimulating  food  with  practically  no 
chance  to  use  her  muscles;  or  we  can  make  quite  a  different  animal 
of  her  by  changing  her  life  and  food.     We  can  control  the  conditions 


ACQUIRED    RESISTANCE.  257 

of  life  among  animals  far  better  than  we  can,  or  will,  those  of  our 
own  life.  In  the  conditions  of  civilized  life  each  individual  de- 
mands his  personal  freedom  in  regard  to  matters  regulating  his  own 
affairs,  and  he  absolutely  refuses  to  be  guided  by  rules  and  regula- 
tions, even  though  he  may  know  them  to  be  for  his  best  physical 
good.  No  matter  how  good  rules  for  living  our  physiologists  may 
make,  they  cannot  force  people  to  adopt  them.  But  the  farmer  has 
absolute  control  over  the  life  conditions  of  his  stock.  He  can 
regulate  their  life  as  suits  him,  and  he  can,  if  he  will,  work  out  among 
cattle  the  problem  of  health  and  disease  as  it  cannot  be  worked  out 
among  men.  He  may,  by  breeding,  produce  animals  with  some 
valuable  feature  most  extremely  developed,  but  in  so  doing  he 
must  remember  that  he  is  producing  abnormal  animals  that  are 
likely  to  have  little  resisting  power  against  disease.  He  may  feed 
them  with  stimulating  food  and  force  them  in  lines  which  suit  him; 
but  he  must  bear  in  mind  that  there  is  a  limit  to  the  possibilities, 
since  all  of  these  methods  of  treatment  lead  to  abnormal  conditions 
and  to  greater  liability  to  disease. 

The  adoption  of  precautions  for  preventing  the  distribution  of 
disease  germs  is  doubtless  a  matter  of  very  great  significance;  but  of 
more  significance  still  is  the  endeavor  so  to  modify  the  conditions  of 
life  as  to  increase  their  resisting  power  against  these  bacteria.  In 
every  case,  doubtless,  the  plan  adopted  will  be  by  the  way  of.  com- 
promise, and  will  be  such  as  to  give  the  greatest  amount  of  physi- 
cal vigor  consistent  with  the  ends  which  the  farmer  has  in  view  in 
his  use  of  the  animals.  To  turn  them  out  into  the  fields  with  no 
attempt  to  produce  special  types,  and  with  no  high  feeding,  would 
doubtless  produce  a  vigorous  breed,  but  it  would  not  produce  milk. 

ACQUIRED  RESISTANCE. 

Some  species  have  a  perfect  resistance  to  the  diseases  that  other 
species  will  take,  a  condition  called  race  immunity.  Some  individuals 
will  resist  a  disease  that  others  of  the  same  species  cannot  resist,  and 
this  is  individual  immunity.  An  individual  may  also  develop  a 
resistance  to  a  disease  which  he  did  not  at  first  possess.     This  is 


258  PARASITIC    BACTERIA. 

acquired  immunity.  It  has  long  been  known  that  if  a  person  has  one 
attack  of  certain  diseases  and  recovers,  he  is,  for  a  time  at  least, 
protected  from  a  second  attack  of  the  same  disease.  It  is  not  common, 
for  example,  to  have  scarlet  fever  twice,  and  the  same  is  true  of  a 
number  of  other  diseases.  This  acquired  immunity  is,  however, 
quite  variable.  In  some  cases  it  is  almost  a  perfect  protection  for 
life,  or  at  least  for  many  years.  With  other  diseases  it  is  weaker, 
affording  only  a  partial  protection  and  lasting  only  a  few  months  or 
perhaps  only  a  few  weeks.  The  question  of  what  causes  this  ac- 
quired immunity  is  closely  akin  to  what  causes  race  or  individual 
resistance.  Doubtless  the  two  are  closely  related  and  are  probably 
attributable  to  the  same  general  cause.  For  our  purpose  it  is  only 
necessary  to  know  that  recovery  from  one  of  these  diseases  leaves 
the  individual  with  his  body  filled  with  substances  capable  of  re- 
sisting the  kind  of  bacteria  that  produced  the  disease.  As  long 
as  these  resisting  substances  are  present  the  individual  will  have 
immunity. 

It  is  somewhat  surprising  that  recovery  from  a  mild  attack  of  one 
of  these  diseases  gives  as  much  immunity  as  recovery  from  a  severe 
attack.  Hence,  with  this  principle  in  mind,  the  question  has  arisen 
whether  it  may  not  be  possible  to  give  an  individual  a  mild  case  of 
some  of  the  more  dangerous  diseases  in  order  to  give  him  power  to 
resist  the  more  severe  and  perhaps  fatal  types.  This  was  first  done 
in  the  case  of  smallpox,  which  has  for  a  century  been  fought  upon 
this  principle,  since  the  vaccination  pustule  seems  to  be  essentially  a 
mild  type  of  smallpox.  Hence,  when  a  person  is  vaccinated,  he  is 
given  a  mild  form  of  smallpox,  and  this  guards  him  from  a  more 
severe  attack.  That  vaccination  is  a  protection  against  smallpox 
is  pretty  generally  admitted  to-day,  although  some  deny  its  power. 

But  whatever  be  the  facts  in  regard  to  smallpox,  there  is  no 
doubt  at  all  in  regard  to  the  successful  application  of  this  principle 
to  other  diseases.  Pasteur  was  the  first  to  attempt  an  application 
of  this  principle  to  a  disease  other  than  smallpox.  He  was  at  the 
time  working  upon  a  serious  disease  of  cattle — anthrax;  one  that 
is  practically  always  fatal.  He  argued  that  if  he  could  find 
means  for  producing  a  mild  type  of  the  disease,  he  might  protect 


ACQUIRED    RESISTANCE.  259 

the  herds  from  the  severe  and  fatal  infection.  But  to  induce  a 
mild  attack  was  not  easy.  It  would  do  no  good  to  inoculate  an 
animal  with  a  small  number  of  the  bacilli  which  produce  the  disease, 
for  these,  by  multiplying,  would  soon  become  so  numerous  as  to 
bring  about  a  severe  attack  of  the  disease.  It  was  evident  that 
this  end  could  be  reached  only  by  weakening  the  power  of  the  dis- 
ease-producing bacilli.  After  continued  experimenting  he  finally 
accomplished  his  purpose  by  cultivating  the  bacilli  at  a  temperature 
somewhat  above  that  at  which  they  make  their  best  growth.  By 
the  use  of  a  temperature  of  io8°  F.  he  obtained  cultures  that  were 
so  weak  as  to  be  unable  to  produce  a  fatal  disease,  even  in  suscep- 
tible animals. 

After  reaching  this  result  Pasteur,  by  an  ever  memorable 
experiment,  demonstrated  to  the  world  the  possibility  of  combating 
infectious  diseases  by  the  use  of  what  are  now  known  as  weakened 
cultures.  He  inoculated  half  of  a  lot  of  fifty  susceptible  animals, 
including  cattle  and  sheep,  with  his  weakened  virus.  The  animals 
were  slightly  indisposed,  but  suffered  no  evil  consequences.  In  a 
few  days  he  inoculated  them  with  a  second,  stronger  culture,  with 
a  like  harmless  result.  Having  thus  prepared  these  test  animals, 
he  summoned  to  a  public  experiment  an  assemblage  of  noted  men 
in  Paris,  and,  in  the  presence  of  them  all,  inoculated  the  entire 
fifty  animals  with  the  strong  infectious  material  taken  from  an 
animal  dead  from  the  disease.  Two  days  later  the  company 
assembled  again  to  find  all  of  the  unprotected  animals  either  dead  or 
dying  from  a  violent  case  of  anthrax,  while  of  the  protected  animals 
not  a  single  one  showed  the  slightest  evil  result  from  the  inoculation. 

A  more  beneficial  discovery  has  hardly  ever  been  made.  From 
the  date  of  Pasteur's  experiment  a  constant  succession  of  bacteri- 
ologists has  been  trying  to  apply  the  same  principle  elsewhere. 
We  cannot  here  attempt  to  follow  the  development  of  the  work, 
but  can  only  state  that  practical  results  of  the  utmost  value  have 
been  obtained.  It  has  not  been  found  possible  to  use  just  the  same 
method  in  other  diseases  that  Pasteur  used,  but  by  a  modification 
of  it,  or  by  others  that  have  come  from  it,  it  has  been  found  possible 
to  withdraw  the  terrors  from  some  of  the  most  dreaded  diseases. 


26o  PARASITIC    BACTERIA. 

The  human  diseases  diphtheria,  lock-jaw,  bubonic  plague,  cholera, 
and  hydrophobia  have  either  been  mastered  or  at  least  mitigated 
by  discoveries  that  have  come  from  the  study  which  Pasteur  started ; 
while  among  animals  at  least  two  diseases  are  controlled  by  prevent- 
ive inoculation — anthrax  and  black  leg.  Some  success  also  has 
attended  similar  methods  with  tuberculosis. 

MICROORGANISMS  THE  CAUSE  OF  DISEASES. 

The  studies  of  the  last  twenty-five  years  have  demonstrated  that 
the  majority  of  diseases  in  animals  and  plants  are  produced  by  the 
growth  of  parasites  of  some  kind,  and  mostly  by  what  may  be  called 
microorganisms.  Bacteria,  yeasts,  and  molds  are  all  concerned  and, 
in  addition,  some  diseases  are  produced  by  microscopic  animals. 
The  subject  of  germ  diseases  has  become  one  of  very  wide  range 
and  cannot  be  considered  to  any  great  length  in  this  work.  The 
discussion  of  such  diseases  among  animals  belongs  to  veterinary 
medicine,  and  of  those  among  plants  to  botany.  In  our  general 
consideration  of  microorganisms  as  related  to  agriculture,  we  can 
review  only  the  important  principles  involved,  and  give  a  brief 
survey  of  the  more  important  diseases. 


CHAPTER  XVIII. 
TUBERCULOSIS. 

Of  all  germ  diseases  there  is  none  so  widely  distributed  as 
tuberculosis.  Not  only  is  it  of  great  significance  from  the  stand- 
point of  human  health,  but  it  is  the  one  disease  of  domestic  animals 
which  demands  almost  universal  attention  and  interest.  Tubercu- 
losis among  cattle  forms  one  of  the  most  serious  problems  of 
agriculture. 

Cause  of  the  Disease. — The  organism  which  produces  this 
disease  was  first  described  by  Koch  in  an  epoch-making  monograph 
published  in  1882.  Koch  first 
isolated  the  bacterium  from  the 
sputum  of  consumptive  patients, 
and  subsequently  found  it  in 
abundance  in  animals  suffering 
from  certain  diseases  now  known 
to  be  forms  of  tuberculosis.  The 
organism  itself  appears  com- 
monly in  the  form  of  a  short, 
slender  rod  (Fig.  51).  Although 
commonly  called  Bacillus  tuber- 
culosis, it  cannot  properly  be 
called  a  Bacillus,  since  it  pos- 
sesses no  flagella.*  (see  page 
12).  Although  this  organism 
does  not  form  spores,  it  has  a 

considerable  resistance  against  heating  and  even  drying.     It  may  be 
dried,  and  yet  remain  alive  for  months,  without  losing  its  power  of 

*  Recent  studies  have  shown  that  the  organism  may  show  blanching 
which  is  not  the  case  with  any  true  bacteria.  It  has  been  suggested  that 
it  should  be  placed  in  a  special  family  named  Myxobacteriae,  It  will 
doubtless  retain  the  name  B.  tuberculosis. 

261 


Fig.  51. — Tuberculosis  bacillus,  a,  in  a 
bit  of  animal  tissue;  b,  showing  irregu- 
larities resembling  spores;  c,  typical  ap- 
pearance of  the  bacilli  from  ordinary  cul- 
tures. 


262  TUBERCULOSIS. 

growth.  It  will  withstand  the  heat  of  140°  for  fifteen  minutes  or 
more,  and  under  some  conditions  a  considerable  higher  heat.  A  few 
minutes'  heating  at  1 75°  will,  however,  kill  it.  Unlike  many  bacteria 
the  tuberculosis  organism  is  quite  limited  as  to  the  conditions  under 
which  it  can  grow,  and  an  understanding  of  these  conditions  is  of 
the  greatest  importance  in  comprehending  the  problems  of  its 
distribution.  The  temperature  limits  within  which  its  develop- 
ment is  possible  are  quite  narrow.  It  grows  best  at  a  temperature 
between  96°  and  105°  F.,  but  it  will  grow  more  slowly  at  a  tempera- 
ture as  low  as  84°  F.  Below  this  it  will  not  multiply  at  all.  At 
first  it  was  supposed  that  it  would  not  grow  in  any  artificial  medium 
which  could  be  prepared  in  the  laboratory.  In  his  original  experi- 
ments Koch  was  obliged  to  use  coagulated  blood  serum  as  a  culture 
medium.  It  is  now  found  that  it  can  live  and  flourish  in  a  variety 
of  culture  media,  provided  a  certain  amount  of  glycerin  be  added. 
It  was  at  first  said  to  be  a  perfect  parasite,  by  which  term  is  meant 
that  it  would  not  live  under  any  conditions  except  those  of  a  warm- 
blooded animal,  demanding  both  a  temperature  and  a  medium 
equivalent  to  the  blood  of  such  an  anmial.  But  here,  too,  bacteriolo- 
gists have  changed  their  views,  for  the  tubercle  bacillus  will  grow 
in  many  laboratory  media  and  under  conditions  very  different 
from  those  of  the  living  body. 

The  facts  just  enumerated  are  of  the  greatest  significance  as 
indicating  the  possibilities  of  distribution  of  this  disease.  If  the 
bacillus  can  live  outside  the  bodies  of  animals,  we  may  look  to 
various  places  in  nature  as  a  source  of  infection,  but  if  it  demands 
for  its  existence  conditions  of  the  living  body,  we  may  look  to 
animals  alone  for  its  source.  Now,  although  it  can  grow  under  con- 
ditions quite  different  from  those  of  the  living  body,  nevertheless,  so 
far  as  our  present  knowledge  goes,  the  tubercle  organism  does  not 
grow  outside  the  bodies  of  animals  under  any  normal  conditions.  It 
does  not  grow  in  water  or  in  milk,  two  facts  of  the  utmost  im- 
portance in  understanding  its  distribution.  It  is  true  that  the 
bacillu^may  frequently  be  found  alive  outside  the  bodies  of  animals. 
It  occurs  in  sputum,  in  milk,  in  water,  in  dust,  etc.,  but  in  these 
media  it  does  not  multiply,  at  least  under  any  conditions  to  which 


TUBERCULOSIS.  263 

they  are  normally  subjected,  and  we  must  therefore  conclude  that 
its  multiplication  is  confined  to  the  bodies  of  animals.  While  it  can 
flourish  in  the  artificial  media  of  the  laboratory,  when  kept  at 
special  temperatures,  it  does  not  flourish  in  nature,  outside  the 
bodies  of  animals  upon  which  it  lives  as  a  parasite. 

Animals  Subject  to  the  Disease. — Besides  living  in  man  the 
organism  can  flourish  in  the  bodies  of  cattle,  hogs,  dogs,  cats,  monkeys, 
rabbits,  guinea-pigs,  and  some  other  animals.  In  all  these  it  pro- 
duces very  similar  symptoms,  differing  slightly,  of  course,  in  the 
different  animals.  The  characteristic  feature  of  the  disease  is  the 
production  of  tubercles — swollen  masses  of  tissue — which  eventually 
break  down  into  a  cheesy  mass.  These  tubercles  may  appear  at 
almost  any  part  of  the  body.  Of  all  the  animals  the  guinea-pig  is 
the  most  delicately  susceptible  to  the  bacillus.  An  extremely  small 
infection  will  produce  the  disease  in  the  guinea-pig,  and  for  this 
reason  these  animals  are  used  in  experiments  to  test  the  presence  of 
the  bacillus.  A  little  suspected  milk  inoculated  under  the  skin  of 
the  guinea-pig  will  produce  the  disease  inevitably,  if  only  the  smallest 
number  of  virulent  germs  are  present.  Besides  these  mammals  a 
number  of  birds  show  a  similar  disease,  with  a  similar  bacillus 
present  in  the  infected  organs.  The  bacillus  in  birds  is,  however, 
in  some  respects,  slightly  different  from  that  in  men  and  cattle,  and 
is  frequently  regarded  as  a  different  type  of  the  organism. 

Most  parasitic  bacteria  are  able  to  grow  only  on  certain  parts  of 
the  body,  diphtheria  commonly  in  the  throat,  cholera  in  the  intestine, 
etc.  But  the  tubercle  bacillus  can  live  in  almost  any  part.  It  is 
found  in  the  intestinal  organs,  in  the  lymphatic  glands,  in  the  lungs,  in 
the  bones,  in  the  joints,  in  the  kidneys,  in  the  skin,  and,  in  short, 
almost  anywhere.  When  occurring  in  the  different  organs  in  man 
it  receives  different  names;  consumption,  scrofula,  lupus,  hip  disease, 
nephritis  are  some  of  its  common  names. 

Resistance  Against  Tuberculosis. — Although  this  organism 
can  attack  almost  any  part  of  the  body,  it  is  also  certain  that  the 
body  has  a  strong  resisting  power  against  it.  It  by  no  means  follows 
that  a  person  will  take  the  disease  because  some  of  the  bacilli  find 
entrance  into  his  body.     On  the  contrary,  as  a  general  rule^  they 


264  TUBERCULOSIS. 

are  soon  overcome  by  the  body  resistance.  Careful  study  has  shown 
that  most  people,  by  the  time  they  have  reached  twenty-five  years  of 
age,  have  not  only  been  exposed  to  the  disease,  but  have  had  mild 
attacks,  from  which  they  have  completely  recovered.  By  building 
up  a  proper  physical  vigor,  an  individual  may  successfully  combat 
these  parasites.  Plenty  of  wholesome,  but  not  too  rich  food, 
exercise,  life  out  of  doors  as  much  as  possible,  sleeping  in  rooms 
with  windows  open  in  winter  as  well  as  in  summer,  and  deep  breath- 
ing exercises,  by  means  of  which  the  lungs  are  filled  with  fresh  air, 
are  the  means  by  which  such  resistance  can  be  developed  and  main- 
tained. All  of  these  conditions  are  usually  within  the  reach  of 
everyone,  so  that  there  is  no  reason  why  a  person  who  will,  cannot 
develop  a  high  resistance  against  this  dreaded  disease. 

ARE  BOVINE  AND  HUMAN  TUBERCULOSIS  THE 

SAME? 

Apart  from  its  relation  to  the  human  being,  the  farmer  is  most 
naturally  interested  in  this  disease,  since  it  attacks  his  cattle. 
Bovine  tuberculosis  is  one  of  the  most  serious  dangers,  and  threatens 
the  continuance  of  dairying. 

The  significance  of  the  question  whether  human  and  bovine 
tuberculosis  are  identical  is  self-evident.  If  the  two  are  the  same,  it 
will  follow  that  the  disease  may  pass  from  animals  to  man;  if  they 
are  not  identical  such  transmission  is  impossible.  For  some  fifteen 
years  after  the  cause  of  the  disease  was  discovered,  no  question  was 
raised  as  to  their  identity.  Both  diseases  are  produced  by  bacteria 
that  appear  identical,  and  that  they  were  the  same  was  taken  for 
granted.  In  1900,  however.  Prof.  Koch  raised  the  question  whether 
they  were  not  distinct,  and  gave  experiments  to  show  that  the 
human  bacillus  does  not  produce  the  severe  bovine  tuberculosis 
when  inoculated  into  cattle.  The  question  caused  intense  interest 
and  much  discussion,  and  in  spite  of  many  experiments  designed  to 
settle  the  matter,  there  is  still  some  dispute.  A  fair  summary  of  the 
facts  as  they  appear  to  the  majority  of  bacteriologists  to-day  is  as 
follows : 


BOVINE   TUBERCULOSIS.  265 

Both  bovine  and  human  tuberculosis  are  caused  by  a  bacterium 
that  has  great  similarity  in  the  two  animals.  But  there  are  slight 
differences  between  them,  both  in  microscopic  appearance  and  in 
methods  of  growth,  sufficient  to  make  it  necessary  to  recognize  them 
as  somewhat  different  types.  When  inoculated  into  animals,  the 
organism  from  the  bovine  source  proves  to  be  more  virulent  than 
the  one  from  the  human  source.  The  human  bacillus,  when 
inoculated  into  cattle,  generally  produces  only  a  slight  trouble, 
while  the  bovine  bacillus  is  apt  to  bring  about  a  progressive  case  of 
the  disease  of  very  serious  character.  What  effect  the  bovine 
bacillus  has  when  inoculated  into  man  cannot  yet  be  told  from 
direct  experiment,  but  there  appear  to  be  a  number  of  tolerably 
sure  cases  of  accidental  inoculation  of  human  beings  with  the  bovine 
bacillus  that  have  been  followed  by  a  development  of  the  disease. 
The  general  conclusion  is  that,  although  the  two  are  slightly  different, 
each  may  produce  the  disease  in  the  other  animal,  and  that  the 
disease  is,  therefore,  transmissible  from  animals  to  man.  While  the 
conclusion  is  still  doubted  by  Prof.  Koch,  it  is  accepted  by  most 
other  bacteriologists.  Whether  the  bovine  bacillus  is  more  virulent 
for  man  than  is  the  human  bacillus,  as  it  is  for  other  animals,  is  by  no 
means  settled.  Furthermore,  it  is  pretty  generally  agreed  that 
human  tuberculosis  comes  more  often  from  human  sources  than 
from  cattle. 

BOVINE  TUBERCULOSIS. 

In  recent  years,  owing  largely  to  the  feeding  of  swine  with 
creamery  refuse,  the  disease  is  coming  to  be  somewhat  common 
among  swine.  But  it  is  among  cattle  that  the  trouble  is  most 
widely  distributed  and  of  the  most  serious  import.  In  cattle  it 
attacks  chiefly  the  glands  of  the  neck,  the  glands  of  the  intestinal 
tract,  and  the  lungs.  It  may  be  located  in  the  udder;  and  in  these 
cases  the  milk  of  the  animal  becomes  a  source  of  danger.  For- 
tunately, the  percentage  of  cases  of  udderdisease  is  comparatively 
small.  In  cattle  it  rarely  attacks  the  bones,  joints,  or  muscles. 
23 


266  TUBERCULOSIS. 

METHODS  OF  DISTRIBUTION. 

Tuberculosis  is  contagious.  By  this  is  meant  that  the  relation 
of  the  bacillus  to  the  animal  is  such  that  there  is  an  easy  means 
of  communication  between  one  animal  and  another  under  the 
ordinary  conditions  of  life.  The  knowledge  of  this  fact  in  regard 
to  human  consumption  has  been  of  great  value,  since  it  has  been 
followed  by  a  steady  decline  in  the  amount  of  the  disease.  Such 
knowledge  has  not  yet  reduced  the  amount  of  bovine  tuberculosis. 

We  can  easily  understand  the  methods  of  contagion  when  we 
remember  that  the  bacilli  are  discharged  from  any  of  the  open 
tubercles.  If  the  disease  is  located  only  in  an  internal  lymphatic 
gland  it  may  not  result  in  breaking  down  the  gland,  and  there  may 
be  no  discharge.  Under  this  condition  there  is  no  contagion  from 
one  animal  to  another.  But  if  it  be  located  in  the  lungs  the  bacilli 
will  be  discharged  into  the  air  passages  and  pass  through  the  trachea 
into  the  mouth.  They  will  then  infect  all  the  discharges  from  the 
mouth  and  nose.  It  is  true  that  the  cow  does  not  expectorate,  but 
by  putting  her  nose  in  the  drinking  trough  she  will  be  sure  to  con- 
taminate the  drinking-water,  and  when  she  licks  another  animal,  as 
she  will  be  sure  to  do  if  she  stands  near  others,  she  will  leave  some  of 
the  bacilli  clinging  to  the  second  individual,  ready  to  begin  their  mis- 
chief if  they  chance  to  get  carried  to  a  susceptible  part,  as  they  are 
very  likely  to  do  by  being  swallowed.  Moreover,  since  the  cow  does 
not  expectorate,  she  does  swallow  the  secretions  from  her  mouth. 
The  tubercle  bacilli  will  thus  be  carried  to  the  stomach,  and  through 
the  intestine,  from  whence  they  will  be  voided  with  the  excrement. 
If  the  disease  is  located  in  the  intestine  the  bacilli  will  be  sure  to  be 
discharged  with  the  excrement.  In  these  ways  the  excrement  of 
tuberculous  cattle  is  sure  to  be  impregnated  with  the  bacilli.  Now 
the  conditions  of  the  ordinary  cow  stall,  even  in  the  best  cow  barn, 
are  such  as  to  make  it  almost  inevitable  that  the  infectious  material 
will  soon  be  distributed  through  the  whole  barn.  The  excrement 
may  be  carried  over  the  floor,  perhaps,  for  some  distance  to  the 
opening  used  for  its  exit,  and  the  farmer's  boots  will  always  collect 
more  or  less  and  carry  it  through  the  barn.     The  particles  adhering 


METHODS    OF    DISTRIBUTION.  267 

to  his  boots  will  be  sufe  to  be  knocked  off  when  dry,  and  will  thus 
be  carried  everywhere  that  the  farmer  goes.  They  will  be  certain 
to  be  dislodged  near  a  healthy  cow  and  may  become  mixed  with 
her  food  which  is  commonly  thrown  on  the  floor  in  front  of  her;  or  the 
particles  may  become  dry  and  be  distributed  through  the  barn  as 
dust.  In  short  it  is  inevitable  that  the  bacilli  voided  with  the 
excrement  will  in  time  come  in  contact  with  every  healthy  animal 
kept  in  the  same  barn. 

Once  distributed  from  infected  animals,  the  bacilli  may  find 
entrance  into  the  healthy  animals,  by  a  variety  of  channels.  Some 
find  entrance  to  the  lungs,  either  by  the  dust  particles  or  by  the 
bacilli-laden  moisture  drops  from  coughing  animals,  which  are 
breathed  by  healthy  animals.  The  bacilli  which  find  their  way 
into  the  watering  trough  will  be  swallowed,  and  the  same  will  be 
true  of  those  which  the  animal  takes  into  its  mouth  by  licking  its 
infected  neighbor.  These  two  means  of  entrance  are  doubtless 
responsible  for  most  cases  of  bovine  tuberculosis,  and  it  is  very 
easy  to  understand  how  a  single  diseased  animal  in  a  barn  may, 
in  time,  infect  most  of  the  herd. 

Abundance  of  Bovine  Tuberculosis. — Tuberculosis  is  widely 
distributed  among  cattle,  although  it  is  by  no  means  universally 
found  in  countries  where  cattle  are  kept.  It  is  said  not  to  occur 
in  Africa,  and  until  recently  it  has  been  absent  from  China  and 
Japan,  having  lately  been  introduced  with  imported  cattle.  In 
the  western  part  of  the  United  States,  among  the  cattle  living  out 
of  doors  most  of  the  time,  it  is  rare  or  absent.  In  general  it  is  most 
abundant  in  localities  where  the  cattle  are  housed  for  a  considerable 
part  of  the  year.  It  is  consequently  most  abundant  in  northern 
countries,  and  appears  to  be  most  widely  distributed  in  northern 
Europe. 

It  is  practically  impossible  to  state  the  percentage  of  cattle 
suffering  from  tuberculosis.  Among  the  animals  examined  in  the 
slaughter  houses  of  Denmark  it  has  sometimes  appeared  that  more 
than  half  of  the  cows  are  tuberculous.  From  these  high  figures 
the  percentage  has  ranged  down  to  lo  per  cent,  or  even  lower  in 
some  places,  and,  in  fact,  is  so  variable  that  no  general  averages 


268  TUBERCULOSIS. 

are  of  any  significance.  In  the  United  States  the  results  differ 
so  widely  that  figures  have,  as  yet,  little  value.  Sometimes  every 
animal  in  a  herd  is  found  to  be  tuberculous,  while  other  whole 
herds  are  entirely  exempt.  In  the  eastern  States  the  percent- 
age is  large,  and  in  some  localities  it  appears  to  approach  the 
figures  given  for  Denmark.  When  the  numbers  of  infected  animals 
in  a  herd  range  from  o  to  loo  per  cent,  it  is  evident  that  no  resulting 
average  would  be  of  any  significance. 

Increase  of  the  Disease. — Is  bovine  tuberculosis  on  the 
increase  ?  Statistics  are  so  uncertain  as  to  make  any  conclusion 
difficult.  Certainly  we  hear  much  more  of  the  disease  than  we 
did  a  few  years  ago,  and  the  percentages  reported  to-day  are  much 
higher.  The  knowledge  of  the  disease  is,  however,  of  very  recent 
date,  and  the  increasing  interest  in  the  subject  has  caused  a  more 
and  more  careful  inspection  of  slaughtered  animals,  which  has 
resulted  in  a  constant  increase  in  the  number  of  reported  cases. 
Even  in  the  same  slaughter  houses  and  under  the  same  management, 
the  percentage  of  tuberculous  animals  reported  has  been  increasing 
year  by  year  in  such  a  way  as  to  seem  to  indicate  an  alarming 
increase  in  the  last  fifteen  years.  But  a  considerable  part  of  this 
increase  is  clearly  due  to  increased  experience  and  carefulness  in 
inspection.  To  what  extent  this  factor  explains  it,  and  to  what 
extent  there  is  an  actual  increase  in  the  disease,  no  one  can  pretend 
to  say.  It  is,  therefore,  impossible  to  state  whether  bovine  tubercu- 
losis is  rapidly  or  slowly  increasing  or  remaining  stationary. 
But  taking  all  facts  together,  the  practical  uniformity  with  which 
the  percentage  of  reported  cases  has  increased  in  the  last  years, 
has  led  to  the  general  belief  that  the  disease  is  actually  and  some- 
what rapidly  increasing  among  our  herds. 

But  although  no  definite  statistics  can  be  given,  either  as  to 
the  prevalence  of  the  disease  or  its  increase,  bovine  tuberculosis 
is  abundant  enough.  It  presents  a  very  serious  problem  to  the 
farmer.  Entirely  independent  of  the  question  of  its  relation  to 
human  tuberculosis,  the  disease,  as  it  exists  among  cattle,  is  a 
menace  to  the  dairy  industry.  The  amount  of  financial  injury 
that  it  does  to  the  farmer  each  year  is  very  great — far  in  advance  of 


THE    COMBAT   AGAINST   BOVINE    TUBERCULOSIS.  269 

any  other  disease.  The  insidiousness  with  which  it  finds  its  way 
into  and  spreads  through  the  whole  herd,  even  before  the  farmer  is 
aware  of  its  presence,  the  large  number  of  cattle  rendered  worthless 
through  its  agency,  especially  among  high-bred  and  valuable  ani- 
mals, the  suspicion  which  it  throws  upon  the  milk-supply,  the 
injury  that  it  does  to  the  animal  which  is  to  be  used  as  food,  the 
great  cost  of  tuberculosis  legislation  by  the  different  States,  all 
these  serve  to  emphasize  the  seriousness  of  the  problem.  Nothing 
can  be  of  more  importance  to  the  farmer  than  the  discovery  of  some 
means  of  controlling  this  disease.  Legislation  designed  to  control 
it  has  been  adopted  by  most  states  in  Europe  and  America,  but 
such  legislation  has  usually  had  in  mind  the  protection  of  the  public 
rather  than  the  assistance  of  the  farmer. 

THE  COMBAT  AGAINST  BOVINE  TUBERCULOSIS. 

Resistance  of  Cattle. — The  foundation  of  a  successful  contest 
against  the  disease  is  a  herd  of  animals  in  a  proper  condition  to 
resist  it.  This  side  of  the  question  is  too  commonly  neglected, 
and  nearly  all  of  the  attempts  made  to  combat  the  disease  have 
been  directed  solely  toward  devising  measures  for  preventing  the 
distribution  of  the  bacillus.  It  is,  however,  impossible  absolutely 
to  prevent  the  bacillus  from  being  distributed  by  diseased  ani- 
mals, and  occasional  infection  will  occur  in  spite  of  all  preventive 
measures.  Without  some  efforts  directed  toward  producing  a 
healthy  herd  of  resisting  animals,  it  is  quite  certain  that  the  endeavor 
to  prevent  the  distribution  of  the  disease  will  be  unsatisfactory. 

It  is  doubtless  much  more  easy  to  give  the  farmer  directions 
looking  toward  the  prevention  of  the  spread  of  the  bacillus,  than 
it  is  to  instruct  him  how  he  may  increase  the  resisting  power  of 
his  animals.  But  nevertheless  some  suggestions  may  be  made 
which,  if  carried  out,  will  certainly  improve  the  conditions  and 
induce  better  health  and,  hence,  greater  resisting  powers.  There 
is  little  doubt  that  in  a  majority  of  cases  the  cattle  need  more  air. 
Too  many  are  crowded  together  in  a  small  space  in  the  winter  season 
and  there  is  too  little  ventilation  of  the  cow  stalls.     In  the  attempt 


270  TUBERCULOSIS. 

to  keep  animals  warm,  they  have  been  too  closely  shut  up  in  badly 
ventilated  rooms,  and  they  breathe  the  warm  air  over  and  over  again. 
Such  a  condition,  wholly  independent  of  the  tubercle  bacilli  which 
might  be  present,  has  a  debilitating  effect  upon  cattle,  just  as  it 
does  on  men.  Too  frequently,  even  on  the  better  farms,  the  cattle 
are  shut  up  in  the  stalls  early  in  the  fall,  are  not  allowed  to  go  out 
during  the  long  months  of  the  winter,  and  never  get  a  breath  of 
fresh  air.  Sometimes  the  case  is  even  worse  than  this,  for  many  cows 
are  thus  shut  up  as  soon  as  they  begin  to  produce  milk,  and,  winter 
and  summer  alike,  remain  in  close,  poorly  ventilated  rooms.  To 
protect  his  cattle  from  cold  the  farmer  makes  his  cow  barn  too  warm 
and  allows  it  too  little  air.  To  save  trouble  he  keeps  the  cows  housed 
all  the  time,  with  no  out-of-door  air;  and  to  save  expense  he  crowds 
them  together  in  the  smallest  amount  of  space.  These  facts 
show  why  so  many  animals  yield  to  tuberculosis  in  the  colder  coun- 
tries. Warm  rooms  and  a  close  crowding  of  the  animals  may 
result  in  a  saving  of  food,  but  it  invites  the  spread  of  tuberculosis 
if  it  once  gains  access  to  a  single  animal.  In  the  human  race  it  is 
well  known  that  the  best  protection  against  the  disease,  and  the 
best  remedy  for  it  after  it  has  once  started,  is  out-of-door  life. 
Doubtless  the  same  is  true  of  cattle,  but  this  fact  has  been  almost 
forgotten  in  the  attempt  to  produce  the  most  milk  possible  at  the 
smallest  expense.  The  farmer  may  perhaps  insist  that  such  crowded 
conditions  are  necessary  and  unavoidable  in  the  modern  farm,  but 
he  must  also  remember  that,  whether  necessary  or  not,  they  are 
certainly  inviting  tuberculosis  and  bringing  his  animals  into  a 
condition  where  they  are  sure  to  yield  to  the  infection  the  first  time 
that  chance  brings  the  bacillus  in  their  vicinity.  More  outdoor 
life  and  more  air  are  the  prerequisites  for  a  healthy  herd. 

Anything  which  will  induce  a  vigorous  life  will  decrease  the 
tendency  to  the  disease.  Proper  food  is  an  important  factor  in 
determining  health.  It  may  be  difficult  under  the  conditions  of 
modern  farming  to  allow  the  cattle  to  have  proper  exercise  in  the 
winter,  but  the  lack  of  it  is  certainly  one  of  the  factors  tending  to 
increase  the  liability  to  tuberculosis.  Too  great  attention  paid 
to  the  increase  in  the  yield  of  milk  lessens  the  resisting  power  of 


THE    PROTECTION    OF    THE    HERD.  27 1 

cattle.  Our  agriculturists,  by  overfeeding  with  certain  kinds  of 
food,  and  by  special  high  breeding  for  the  purpose  of  increasing  the 
yield  of  milk,  are  trying  to  turn  an  animal  into  a  milking  machine. 
The  highly  bred  animals  are,  of  course,  useful  for  the  purpose  for 
which  they  are  bred;  but  the  agriculturist  must  remember  that  he 
cannot  turn  his  cow  into  a  simple  milking  machine  without  suffering 
some  evil  results  from  the  change  in  her  nature.  In  short,  if  the 
cattle  owner  will  learn  that  cattle  are  animals  and  not  machines 
and  that  they  need  something  besides  food  and  water  to  keep  them 
active,  he  will  probably  soon  find  the  tendency  to  tuberculosis 
becoming  less. 

THE  PROTECTION  OF  THE  HERD. 

While  the  treatment  of  the  cow  as  an  animal  and  not  a  milking 
machine  must  be  the  foundation  of  a  healthy  herd,  the  care  of  the 
farmer  most  not  stop  there.  The  animals  must  be  guarded  against 
infection  and  to  this  end  much  attention  has  been  given  in  recent 
years. 

The  Tuberculin  Test. — Any  method  of  protecting  a  herd  against 
tuberculosis  must  start  with  some  method  of  detecting  the  disease 
in  animals.  Certain  forms  of  the  disease,  especially  when  in  an 
advanced  stage,  are  easily  discovered  by  clinical  means,  the 
veterinarian  being  able  to  detect  them  by  the  examination  of  the 
cattle.  But  there  are  other  cases  where  no  visible  signs  appear, 
and  these  cannot  be  found  by  clinical  means.  The  tuberculin 
test  has  been  devised  to  meet  this  difficulty  and  to  dectect  even  the 
mildest  cases.  Tuberculin  was  first  prepared  by  Koch.  It  is 
made  by  causing  the  tubercle  bacilli  to  grow  in  a  broth  containing 
glycerin.  While  growing  in  such  a  broth,  the  bacilli  produce  certain 
toxic  products  which  are  soluble  and  which  dissolve  in  the  broth. 
The  material  is  then  treated  in  such  a  way  as  to  remove  the  bacilli, 
and  the  clear,  toxic-holding  solution  is  tuberculin.  Inasmuch 
as  it  does  not  contain  any  living  bacilli,  it  cannot  possibly  cause  the 
disease,  and  its  use  among  animals  cannot  incite  tuberculosis,  as 
has  sometimes  ignorantly  been  claimed. 


272  TUBERCULOSIS. 

Although  containing  no  bacilli,  tuberculin  does  contain  the 
toxins  which  the  bacilli  produce,  and  these  toxins,  if  inoculated 
into  an  animal  in  sufficient  quantity,  would  poison  it.  When 
injected  in  small  quantity,  the  material  has  no  effect  upon  the 
healthy  individual;  but  if  the  individual  is  already  affected  with  the 
disease,  this  inoculation  produces  a  marked  rise  in  temperature, 
which  soon  disappears. 

The  fact  that  the  injection  is  followed  by  a  rise  in  temperature 
makes  it  possible  for  this  material  to  be  used  among  cattle  in  detect- 
ing tuberculosis.  Healthy  animals  fail  to  respond  to  this  inocula- 
tion and  are  wholly  uninjured  by  it.  The  farmer  may,  therefore, 
have  his  herd  tested  with  the  confidence  that  his  healthy  animals  will 
not  suffer  by  the  test.  On  the  other  hand,  the  animals  that  have 
become  infected  with  the  disease  will  show  a  rise  in  temperature, 
and  the  test  will  thus  make  it  possible  to  separate  the  affected 
animals  from  those  that  are  yet  in  health. 

The  accuracy  of  the  test  has  been  the  subject  of  much  dispute. 
It  has  been  found  subject  to  some  error.  If  animals  are  tested 
under  abnormal  conditions,  as,  for  example,  when  in  new  barns,  or  if 
taken  from  a  cattle  car  and  tested  at  once,  even  healthy  animals  may 
respond.  But  when  the  animals  are  in  normal  conditions  the 
healthy  animals  probably  never  respond,  or  at  all  events  so  rarely 
as  not  to  interfere  with  the  accuracy  of  the  test.  Secondly,  some 
animals  very  far  advanced  in  the  disease  fail  to  respond.  These 
cases  are  of  little  importance  since  they  are  commonly  detected 
readily  by  clinical  symptoms.  Thirdly,  all  animals  which  are 
moderately  attacked,  and  all  of  the  very  incipient  cases  of  tuberculo- 
sis, are  detected  by  the  tuberculin.  Even  a  single  minute  tubercu- 
lous gland  is  sufficient  to  cause  a  positive  reaction  to  the  test. 

This  last  fact  forms  at  once  the  strength  and  the  weakness  of  the 
tuberculin  test.  Tuberculin  does  pick  out  with  great  accuracy  all 
mild  cases,  and  clinical  symptoms  will  pick  out  the  rest.  But  this 
test  fails  to  distinguish  between  severe  and  mild  forms,  putting  in 
one  class  the  animal  that  may  have  a  small  tuberculous  gland, 
which  may  heal  in  a  short  time,  and  the  animal  with  a  severe  case  of 
intestinal  tuberculosis  which  is  scattering  bacilli  to  the  great  danger 


THE    PROTECTION    OF    THE    HERD.  273 

of  the  rest  of  the  herd.  Experience  has  shown  that,  of  the  animals 
responding  to  the  test,  some  run  down  rapidly  and  require  slaughter- 
ing in  a  few  weeks,  while  others  wholly  recover,  live  several  years 
of  useful  life,  and  after  death  show,  by  postmortem  examinations, 
that  the  original  tubercle  has  been  healed  and  the  animals  have 
come  again  into  normal  condition.  There  is  thus  a  great  difference 
between  clinical  tuberculosis  and  tuberculin  tuberculosis.  The  former 
results  practically  always  in  the  death  of  the  animal,  the  latter  may 
be  temporary  and  insignificant.  The  former  certainly  is,  and  the 
latter  may  or  may  not  be,  a  source  of  danger  to  the  herd. 

In  the  enthusiasm  which  followed  this  easy  means  of  detection, 
it  was  claimed  that  it  might  be  possible  to  eradicate  tuberculosis 
completely  from  our  herds,  and  some  States  started  upon  a  sweeping 
plan  of  testing  all  cattle  and  slaughtering  immediately  all  animals 
that  responded  to  the  test.  But  this  entirely  too  radical  plan 
proved  quite  impracticable.  Nevertheless,  the  use  of  tuberculin  has 
become  of  great  value  to  the  farmer  in  his  attempts  to  get  rid  of  this 
disease  among  his  cattle. 

The  Preservation  of  a  Healthy  Herd. — If  a  farmer  has  a  herd 
in  which  the  disease  has  not  appeared,  it  is  of  especial  interest  to  him 
to  keep  his  herd  in  this  condition;  for,  once  the  disease  has  entered 
the  herd,  it  is  very  difficult  and  expensive  to  stamp  it  out.  Tuber- 
culosis does  not  develop  spontaneously  in  a  herd  of  animals,  but 
is  always  introduced  from  the  outside.  A  farmer  who  can  raise  his 
own  cattle  and  can  properly  protect  them  from  contact  with  out- 
siders need  have  no  tuberculosis  among  them.  But  to  protect  the 
herd  requires  some  knowledge  and  great  vigilance.  To  prevent 
the  entrance  of  the  disease  into  his  herd  from  without,  the  farmer 
must  exercise  care  in  several  directions. 

First:  In  buying  stock  he  must  be  sure  not  to  purchase  in- 
fected animals.  This  is  perhaps  the  greatest  difficulty,  for  it  is  most 
commonly  by  purchase  that  the  disease  is  introduced  into  a  herd. 
There  is  only  one  way  by  which  he  may  be  sure  that  he  is  not 
purchasing  infected  cattle,  and  this  is  by  a  proper  tuberculin  test, 
under  the  guidance  of  a  reliable  veterinarian.  The  matter  is  made 
more  difficult  by  the  fact  that  after  an  animal  has  been  tested  and 


2  74  TUBERCULOSIS. 

responded,  she  is  for  some  time  protected  from  a  second  test.  A 
dishonest  dealer,  therefore,  may  inoculate  his  cows  privately  and 
then  put  upon  the  market  all  those  that  respond  to  the  test,  knowing 
that  for  some  time  they  will  not  again  respond.  One  thing  is 
certain.  No  farmer  can  be  confident  of  keeping  his  herd  free  from 
thisxiisease  unless  he  can  be  assured  by  the  tuberculin  test  that  he  is 
purchasing  animals  freed  from  every  suspicion  of  the  disease. 

Second:  He  must  prevent  his  cattle  from  associating  with 
strange  cattle.  If  put  out  to  pasture  they  must  be  kept  by 
themselves  and  guarded  against  chance  contact  with  strangers. 
Common  watering  troughs,  in  which  miscellaneous  cattle  are 
watered,  must  be  shunned. 

Third:  He  must  not  feed  his  calves  upon  milk  from  other 
herds.  The  way  in  which  this  is  most  commonly  done  is  by  the  use 
of  skim  milk  returned  from  a  creamery  or  a  separating  station. 
From  such  a  creamery  the  farmer  does  not  get  back  his  own  milk, 
but  always  milk  from  another  source,  and,  if  there  be  a  few  cases 
of  bovine  tuberculosis  of  the  udder  in  the  neighborhood,  the  bacilli 
from  these  animals  will  soon  be  distributed  through  the  separating 
station  over  the  entire  region  contributing  to  the  station.  This  is 
not  mere  theory,  but  positively  ascertained  fact.  To  such  milk  is  to 
be  attributed  the  large  amount  of  tuberculosis  among  swine  in 
recent  years.  The  only  safe  procedure  is  for  the  farmer  either  to 
bring  up  his  calves  on  the  milk  from  his  own  healthy  herd  or  to 
insist  that  all  milk  fed  to  them  shall  first  be  subjected  to  the  process 
of  pasteurizing  or  boiling.  So  convinced  are  the  agriculturists  in 
Denmark  that  this  mixed  milk  is  the  cause  of  much  of  the  bovine 
tuberculosis,  that  a  law  has  been  passed  forcing  the  pasteurization 
of  all  milk  which  is  thus  brought  to  creameries  for  separation  of  the 
cream.  The  farmer  is  thus  protected  from  the  tuberculosis  of  his 
neighbors'  herds. 

Treatment  of  a  Tuberculous  Herd. — There  appear  to  be  four 
general  methods  of  treating  a  herd  after  this  disease  breaks  out  in  it: 

I.  All  advanced  cases  which  are  recognized  as  dangerous  to 
the  public,  including  all  cases  of  udder  tuberculosis,  may  be  removed 
and   the   animals   destroyed,    the   others   being   left   undisturbed. 


I 


THE   PROTECTION    OF    THE    HERD.  275 

This  does  away  with  most  of  the  danger  to  the  public  consuming  the 
milk,  but  does  nothing  toward  eradicating  the  disease  from  the  herd. 

2.  All  animals  that  have  the  disease  as  shown  by  clinical  or 
tuberculin  test  may  be  slaughtered.  The  attempt  to  enforce  by 
law  such  a  treatment  of  the  disease  has  failed  wherever  tried.  It 
involves  too  great  a  loss  and  dooms  to  slaughter  many  animals  in  the 
incipient  stages  of  the  disease  that  might  recover  and  are  still 
useful  animals.  It  is  sometimes  done  voluntarily  by  private  owners 
in  their  determination  to  keep  a  herd  free  from  the  disease.  It 
protects  the  herd  and  the  public  at  the  same  time. 

3.  The  great  losses  that  are  frequently  involved  in  the  slaughter 
of  all  reacting  animals  have  led  to  the  adoption  of  other  plans  for 
freeing  the  herd  of  the  disease  without  such  sacrifices.  A  plan 
was  devised  some  fifteen  years  ago  by  Bang,  consisting  of  separating 
the  reacting  animals  from  the  others.  The  first  step  is  to  detect  by 
tuberculin  all  tuberculous  animals.  The  advanced  cases  are  slaugh- 
tered. The  other  reacting  animals  are  separated  from  the  others 
and  placed  by  themselves,  removed  from  every  possible  contact 
with  the  rest  of  the  animals  in  the  herd.  This  is  not  because  all 
reacting  animals  are  necessarily  sources  of  danger,  but  simply 
because  there  is  no  means  of  determining  when  any  one  of  them 
may  become  a  source  of  danger  to  the  animals  about  it.  The  healthy 
(non-reacting)  animals  are  then  placed  by  themselves,  either  in  a 
new  barn  or  the  old  one  after  it  is  thoroughly  disinfected.  By 
this  means  a  practical  isolation  is  accomplished. 

If  the  farmer  wishes  to  preserve  the  healthy  herd  from  future 
attack,  he  must  take  precautions  to  have  the  isolation  thorough. 
It  may  be  effected  by  simply  building  a  partition  in  his  cattle  shed; 
but  in  this  case  there  should  be  no  door  in  the  partition,  for  that 
would  surely  result  in  a  carrying  of  bacilli  from  one  compartment 
to  the  other.  The  farmer  should  remember  the  facts  already 
pointed  out  as  to  the  methods  of  distributing  bacilli.  If  possible, 
he  should  have  separate  attendants  for  the  two  herds,  and  at  all 
events,  the  boots  worn  in  attendance  on  the  infected  herd  should 
not  be  worn  in  the  shed  occupied  by  the  healthy  animals.  He 
must  remove  all  calves  from  the  infected  herd  a  few  days  after  birth 


276  TUBERCULOSIS. 

and  bring  them  up  on  the  milk  of  the  healthy  herd  alone.  The 
healthy  herd  must  be  tested  every  six  months  or  so,  and  if  any 
show  reaction  they  must  at  once  be  removed  from  the  rest.  The 
tuberculous  herd  may  be  kept  and  milked,  but  the  milk  should  be 
sterihzed.  By  keeping  up  this  procedure  for  a  few  years,  it  is  possible 
to  eliminate  the  infected  animals  and  have  left  a  herd  of  healthy 
animals. 

4.  A  still  more  recent  plan  has  been  widely  adopted  in  Germany 
and  seems  at  present  to  oflfer  the  simplest  and  most  hopeful  solution. 
It  consists  in  separating  all  calves,  as  soon  as  born,  from  their 
mothers,  and  rearing  them  separately  from  the  rest  of  the  herd. 
Tuberculosis  is  not  hereditary,  and  the  calves  of  tuberculous  mothers 
are,  when  born,  free  from  the  disease  (except  in  rare  instances). 
If,  therefore,  they  are  at  once  separated  from  their  mothers,  brought 
up  on  pasteurized  milk,  and  not  allowed  any  possible  contact  with 
the  other  animals,  they  may  be  reared  free  from  the  disease,  becoming 
in  a  few  years  a  herd  of  healthy  cattle,  and  if  they  are  guarded  from 
outside  sources  of  contamination  they  will  continue  to  be  free  from 
the  disease.  Meantime  the  animals  in  the  infected  herd,  whose 
milk  may  be  used  if  throughly  pasteurized,  may  be  slowly  disposed  of, 
while  the  healthy,  growing  herd  gradually  replaces  them  with  the 
smallest  possible  loss  to  their  owner.  Of  course  the  healthy 
animals  must  not  be  allowed  to  enter  the  quarters  formerly  occupied 
by  the  diseased  herd  until  there  has  been  a  thorough  disinfection 
of  the  premises. 

Which  of  these  methods  of  procedure  it  is  best  to  adopt  depends 
upon  circumstances.  If  a  man  has  only  a  small  number  of  animals 
and  only  one  or  two  of  them  are  tuberculous,  his  simplest  plan 
will  be  to  slaughter  the  reacting  animals  at  once.  If  his  herd  is  a 
large  one,  it  is  best  to  build  up  a  healthy  herd  by  one  of  the  methods 
outlined.  He  must  remember  that  a  single  tuberculous  animal 
is  a  menace  to  his  entire  herd  and  he  should  begin  his  fight  against 
the  disease  at  the  very  first  discovery  of  an  affected  animal.  Neglect 
in  using  the  tuberculin  for  fear  that  some  reacting  animals  be  found 
in  the  herd  is  the  height  of  folly.  Half-way  measures  in  handling 
this  subject  are  no  better  than  none. 


FLESH   AND    MILK    FROM    TUBERCULOUS   ANIMALS.  277 

Preventive  Inoculation. — In  recent  years  claims  have  been 
made  that  a  method  of  preventive  inoculation  against  this  disease 
has  been  found  by  using  dried  human  tubercle  bacilli  for  rendering 
cattle  immune.  Considerable  data  have  been  collected  as  to  the 
success  of  this  method,  and  it  seems  pretty  certain  that  a  considerable 
degree  of  immunity  can  thus  be  given  to  cattle.  It  is  yet  too  early 
to  say,  however,  whether  this  procedure  will  ever  be  a  practical 
method  of  handling  the  tuberculosis  problem. 

THE  USE  OF  FLESH  AND  MILK  FROM 
TUBERCULOUS  ANIMALS. 

The  practical  question  of  the  disposal  of  milk  and  flesh  from 
tuberculous  animals  is  constantly  arising.  The  answer  is  clearly 
dependent  upon  whether  the  disease  in  men  and  animals  is  the 
same.  Since  it  is  generally  agreed  that,  if  not  the  same,  the  two  are  so 
nearly  alike  that  they  may  be  transmitted  from  one  to  the  other,  it 
is  the  concensus  of  all  that  the  possibility  of  transference  through 
flesh  or  milk  should  be  guarded  against. 

Flesh. — Tubercular  matter  when  fed  to  susceptible  animals 
may  produce  the  disease  in  the  animal  experimented  upon.  From 
this  it  follows  that,  if  the  human  and  bovine  tuberculosis  are  the 
same  disease,  mankind  may  be  exposed  to  danger  from  eating  the 
flesh  of  tuberculous  cattle.  But  there  is  no  danger  unless  there 
are  tubercle  bacilli  in  the  part  eaten.  The  tuberculous  infection 
of  cattle  is  commonly  in  the  lungs,  intestines  or  lymphatic  glands, 
and  only  rarely  are  the  muscles  affected. 

If  an  animal  has  simply  a  tuberculous  lymphatic  gland,  its 
muscles  are  perfectly  safe  eating,  unless  they  may  have  become 
infected  by  the  knife  of  the  butcher  which  has  previously  cut 
through  some  tuberculous  mass  in  the  animal.  The  danger  to 
man  from  eating  tuberculous  flesh  is  therefore  slight.  Further, 
flesh  is  commonly  cooked  before  it  is  eaten.  Thorough  cooking 
will  destroy  the  bacteria,  but  even  the  moderate  cooking  which 
meat  commonly  receives  is  sufficient  to  destroy  the  bacteria  upon 
its  surface,  although  the  heat  does  not  extend    to    the    interior. 


278  TUBERCULOSIS. 

Inasmuch  as  flesh  is  rarely  the  seat  of  the  tubercular  infection,  and 
accidental  contamination  with  the  butcher's  knife  will  be  on  its 
surface,  cooking  will  almost  always  render  it  harmless,  unless  the 
infection  is  deep-seated.  For  these  reasons  the  flesh  of  animals 
slightly  infected  with  this  disease  need  not  be  condemned  as  food. 
It  is  universally  admitted  that  the  actual  danger  from  this  source  is 
very  small  and  perhaps  does  not  exist  at  all. 

Milk. — The  problem  of  the  use  of  milk  from  tuberculous 
animals  is  a  more  difficult  one  to  settle.  The  milk  of  tuberculous 
cattle  does  not  always  contain  the  bacilli  and  it  is  an  unsettled  question 
whether  it  will  ever  contain  them  unless  the  disease  be  located  in  the 
udder.  At  all  events,  cows  having  tuberculous  udders  (some- 
where about  I  per  cent.)  will  produce  milk  infected  with  tubercu- 
losis bacilli.  That  these  bacilli  are  active  and  vigorous  is  proved 
by  thousands  of  experiments  which  have  shown  that  such  milk 
is  capable  of  producing  tuberculosis  in  guinea-pigs.  It  is  true 
that  the  bacilli  do  not  multiply  in  milk,  but  milk  from  one  cow 
can,  by  being  mixed  with  other  milk,  infect  a  large  amount.  It 
is  possible  that  such  milk  may  be  a  danger  to  the  public  health. 
It  has  been  abundantly  shown  that  market  milk  frequently  contains 
tubercle  bacilli  in  sufficient  quantity  to  produce  an  infection  in 
guinea-pigs,  and  the  same  is  true  of  market  butter.  All  of  these 
facts  certainly  indicate  a  possible  danger  to  the  public  from  this 
source. 

In  regard  to  the  extent  of  this  danger  there  has  been  a  wide 
difference  of  opinion.  It  has  certainly  been  magnified  by  some. 
The  danger  is,  beyond  question,  frequently  overdrawn.  It  is 
sometimes  doubted  that  mankind  can  ever  acquire  tuberculosis 
from  this  source.  Experiment  has  shown  that  large  numbers  of 
the  bacilli  must  be  swallowed  at  once  to  produce  infection  even  in 
susceptible  animals.  The  number  of  bacilli  which  a  person  will 
swallow  with  a  drink  of  milk  will  commonly  be  rather  small,  and 
the  human  individual  has  a  considerable  power  of  resistance 
against  the  disease.  It  is  a  further  fact  that,  although  bovine 
tuberculosis  has  been  increasing,  human  tuberculosis  has  been  con- 
stantly declining  in  recent  years,  and  the  decline  has  been  equally 


I 


FLESH  AND    MILK    FROM    TUBERCULOUS   ANIMALS.  279 

great  in  those  countries  that  use  milk  raw  and  in  those  countries 
that  sterilize  the  milk  before  drinking  it.  This  decrease  in  tuber- 
culosis does  not  apply  to  intestinal  tuberculosis  among  young 
children,  indicating,  possibly,  that  milk  is  a  more  common  source 
of  infection  for  children  than  for  adults.  For  these  various  reasons 
it  is  a  fair  inference  that  the  danger  of  tuberculosis  from  milk  is 
not  very  great  for  adults,  though  it  may  be  considerable  for  young 
children.  It  is  quite  certain  that  for  young  children  it  is  unsafe  to 
resort  to  the  use  of  milk  from  miscellaneous  cows  without  the  pre- 
caution of  pasteurization. 

Certainly  the  logical  method  of  dealing  with  milk  would  be 
to  exclude  from  the  milk-supply  all  milk  from  tuberculous  animals 
or  to  allow  it  to  be  used  only  after  pasteurization.  Only  thus 
could  absolute  safety  be  assured.  But  this  is  quite  impractical, 
if,  indeed,  possible.  A  farmer  who  takes  pride  in  his  dairy  and  in 
furnishing  a  special  quality  of  milk  will  protect  his  cutomers  by 
periodic  testing  of  his  cattle  and  by  the  exclusion  of  all  reacting 
animals.  But  to  enforce  any  regulations  looking  in  this  direction  in 
regard  to  the  public  milk-supply  is  simply  impossible  at  the  present 
time  and  will  remain  so  for  some  time  to  come.  The  end  could 
be  reached  through  the  milk  supply  companies,  by  the  adoption  of 
the  simple  and  inexpensive  process  of  pasteurizing  all  milk  before 
distribution,  and  quite  possibly  such  may  be  the  ultimate  solution 
of  the  problem.  Meantime  the  only  feasible  method  of  treating 
the  matter  is  to  insist  that  the  farmer  shall  rigidly  exclude  from  the 
animals  furnishing  the  milk-supply  all  cows  with  diseased  udders, 
and  to  suggest  to  all  who  have  a  fear  of  using  the  milk  because  of 
the  slight  danger  existing  in  this  food-supply,  that  the  danger  may 
be  wholly  avoided  by  pasteurization. 


CHAPTER  XIX. 
OTHER  GERM  DISEASES. 

ANTHRAX  OR  SPLENIC  FEVER. 

Anthrax  is  a  disease  of  domestic  animals  which  has  been  known 
for  centuries.  It  is  mentioned  in  the  writings  of  Moses,  and  Homer 
refers  to  it  in  the  Ihad.  It  occurs  practically  all  over  the  globe,  in 
all  latitudes  where  cattle  are  kept,  and  seems  to  be  entirely  independ- 
ent of  climate.  Every  country  of  Europe  suffers  from  it.  Germany 
has  lost  some  4,000  cattle  from  this  disease  in  some  years  and 
England  nearly  a  thousand.  In  the  United  States  the  disease 
is  also  frequent,  though  generally  regarded  as  less  common  than 
in  Europe.  Although  widespread,  it  does  not  occur  in  great  numbers 
of  cattle  as  do  some  of  the  other  bacterial  diseases.  It  may  attack 
the  animals  of  a  single  herd  and  produce  much  destruction,  but  it  is 
not  very  contagious  and  does  not  readily  spread  from  herd  to  herd. 

Cattle  and  sheep  are  the  only  animals  in  which  it  normally 
occurs  as  a  spontaneous  infection.  Many  other  animals  are, 
however,  capable  of  infection  with  it.  Horses,  goats,  deer  and  mice 
are  very  subject  to  the  disease,  while  dogs,  cats  and  white  rats  are  not 
susceptible.  The  disease  is  also  found  in  man,  and  is  then  known 
by  various  names,  the  most  common  being  malignant  pustule. 
Mankind  is,  howeVer,  not  one  of  the  very  susceptible  animals  and, 
when  infected  by  a  skin  inoculation,  the  disease  is  quite  apt  to  ^0 
local,  while  in  sheep  and  cattle  it  is  almost  sure  to  run  a  fat- .  course. 

Cause. — The  discovery  of  the  cause  of  this  disease  was  one  of 
the  first  triumphs  of  bacteriology.  Its  exciting  cause  is  a  bacterium, 
Bad.  anthracis,  which,  though  first  seen  in  1849,  ^^^  ^^^  really 
demonstrated  as  the  cause  of  the  disease  until  1875,  by  the  work  of 
Koch,  and  shortly  afterward  by  Pasteur  (Fig.  52).  After  some 
twenty-four  years  of  dispute  the  final  demonstration  was  due  to 

280 


ANTHRAX    OR    SPLENIC    FEVER.  28 1 

such  experiments  as  the  following.  It  was  easy  to  show  by  the 
microscope  that  this  organism  is  present  in  the  blood  of  the  animals 
suffering  from  the  disease,  and  that  a  drop  of  such  blood,  containing 
the  organism,  when  injected  into  healthy  animals  would  inevitably 
produce  the  disease  in  the  inoculated  animals.  But  this  did  not 
necessarily  prove  their  causal  agency,  for  it  was  possible  to  claim 
that  there  were  some  other  poisons  in  such  blood.  For  final  proof 
it  was  necessary  to  separate  the  bacteria  from  the  drop  of  blood, 
cultivate  them,  and  inoculate  animals  with  the  pure  cultures.  At 
the  time  that  this  disease  was  first  being  studied  no  methods  were 
known  of  obtaining  isolated  bacteria  in  pure  cultures,  and  hence 


Fig.  52. — B.  anthracis,  the  cause  of  splenic  fever. 

the  long  dispute.  Pasteur  finally  procured  his  results  as  follows. 
Finding  that  the  bacterium  would  grow  in  a  solution  made  by  steeping 
yeast  in  water,  Pasteur  inoculated  a  sterile  flask  of  such  yeast-water 
with  a  drop  of  anthrax  blood.  In  a  day  or  two  his  flask  was  filled 
with  bacteria  which  had  arisen  from  the  first  by  division.  The 
inoculation  of  a  second  flask  from  the  first  showed  like  results,  and 
by  continuing  such  inoculations  from  flask  to  flask  he  rapidly  got 
rid  of  all  parts  of  the  original  drop  of  blood,  except  such  parts  as  had 
been  multiplying  in  the  flasks.  His  microscope  showed  him  that  the 
only  thing  that  multiplied  and  remained  in  his  later  flasks  were  the 
bacteria  present  in  the  original  drop  of  blood.  Nevertheless,  he 
found  that  though  he  continued  these  inoculations  indefinitely, 
every  flask  was  equally  virulent,  and  a  small  drop  of  the  culture 
would  inevitably  produce  anthrax  in  a  susceptible  animal  in  a  very 
few  hours,  the  development  of  the.disease  being  always  accompanied 
by  the  growth  in  its  blood  of  the  bacilli  in  countless  myriads.  These 
24 


282  OTHER    GERM    DISEASES. 

results  left  no  loophole  for  criticism,  proving  that  this  bacterium  was 
the  cause  of  anthrax,  and  thus  for  the  first  time  demonstrating  that 
an  infectious  disease  was  produced  by  a  bacterium  multiplying 
within  the  body  of  the  animal  in  which  it  grows  as  a  parasite. 

The  bacterium  in  question,  Bad.  anthracis,  is  a  rod  of  moderate 
size  (Fig.  52).  It  multiplies  by  repeated  division,  the  elements  re- 
maining attached  to  form  long  chains.  Sometimes  these  long 
threads  show  no  signs  of  the  divisions,  and  in  certain  media  they 
form  marvelously  twisted  and  contorted  masses.  When  in  an 
active  growing  condition,  this  bacterium  is  readily  killed  by  ordinary 
disinfecting  agents  and  by  a  moderate  heat,  a  temperature  of  about 
160°  F.  easily  destroying  the  rods.  But  it  produces  resisting  spores 
which  can  easily  be  distinguished  inside  the  rods  as  clear,  ghstening 
bodies.  It  is  their  resistance  to  ordinary  agents  that  makes  anthrax 
so  persistent,  and  this  high  resistance  must  be  borne  in  mind  when 
the  attempt  is  made  to  disinfect  a  stable  which  has  been  occupied  by 
an  animal  having  this  disease.  These  spores  will  resist  the  action  of 
5  per  cent,  carbolic  acid  solution  for  half  an  hour,  or  a  i  per  cent, 
solution  of  corrosive  sublimate  for  about  the  same  length  of  time. 
Few  other  living  bodies  can  resist  such  treatment.  The  spores  will 
also  resist  a  temperature  of  about  280°  F.  for  two  or  three  hours. 
When  immersed  in  liquid  they  are  much  more  easily  killed,  since  the 
temperature  of  boiling,  if  maintained  for  a  few  minutes,  is  com- 
monly sufficient  to  destroy  them.  When  dried  the  spores  may 
remain  alive  for  a  long  time,  many  years  at  least,  and  yet  all  the  time 
retain  their  power  of  developing  when  placed  under  proper  condi- 
tions. All  of  these  facts  evidently  make  the  disinfecting  of  an 
infested  locality  a  matter  of  very  great  difficulty. 

Method  of  Infection. — Although  this  disease  is  extremely  fatal, 
animals  affected  rarely  recovering,  it  is  not  particularly  contagious, 
and  is  rarely  commuilicated  directly  from  animal  to  animal.  One 
common  method  by  which  cattle  are  infected  appears  to  be  through 
the  food  which  they  crop  in  the  fields.  It  has  often  been  noticed 
that  the  disease  breaks  out  in  a  herd  shortly  after  it  has  been  turned 
out  into  a  new  pasture.  In  some  of  these  cases  which  have  been 
investigated  the  explanation  is  simple.     In  such  pastures  bodies  of 


ANTHRAX    OR    SPLENIC    FEVER.  283 

animals  dead  from  anthrax  have  been  buried,  and  the  spores  have 
remained  alive  for  many  years.  Now,  although  these  spores  may 
have  been  buried  some  distance  below  the  surface,  they  are  event- 
ually brought  to  the  surface.  One  of  the  means  by  which  they  are 
brought  up  from  under  ground  is  through  the  agency  of  earth- 
worms, and  the  spores  are  later  taken  into  the  stomachs  of  cattle 
feeding  on  the  grass.  These  spores  resist  the  action  of  the  di- 
gestive juices  and  of  the  other  bacteria  present  in  the  intestine,  and 
make  their  way  through  the  intestinal  walls  into  the  body,  producing 
the  disease.  These  facts  readily  explain  many  of  the  phenomena 
connected  with  the  outbreaks  of  epidemics. 

In  other  cases  the  germs  may  find  entrance  through  abrasions  of 
the  skin.  When  thus  introduced  the  bacteria  first  produce  a  simple 
abscess  in  the  skin,  which  soon  turns  into  a  gelatinous  pustule. 
This  pustule  does  not  heal,  and  from  it  as  a  center  the  bacilli  spread 
rapidly  through  the  body,  producing  a  general  disease  which  may 
terminate  fatally.  The  name  malignant  pustule  is  appropriately 
applied  to  this  form  of  disease.  In  susceptible  animals  such  re- 
covery is  very  rare.  In  the  case  of  animals  which,  like  man,  are 
less  susceptible  to  the  disease,  these  abscesses  may  remain  simple 
localized  infections,  eventually  healing  without  spreading  through  the 
body.  There  are  other  modes  of  infection,  but  among  animals  the 
disease  is  most  usually  acquired  through  the  intestine  or  through 
skin  abrasions. 

In  the  body  of  the  infected  animals  the  bacilli  grow  with  great 
rapidity.  An  extremely  small  number  of  them  inoculated  into  the 
body  of  a  sheep  may  produce  its  death  in  about  two  days,  and  after 
death  the  whole  body  is  found  to  be  filled  with  the  bacilli  in  incal- 
culable numbers.  The  disease  is  marked  by  a  high  fever  and  much 
discomfort,  and  after  death  the  most  characteristic  symptom  is  a 
greatly  swollen  spleen,  whence  the  name  splenic  fever.  The  spleen 
is  large,  hard,  and  brittle,  and  contains  enormous  numbers  of  the 
bacilli.  The  blood-vessels  are  also  found  to  be  full  of  them,  and  the 
capillaries  may  literally  be  crammed  with  bacteria. 

This  bacillus  is  extremely  virulent  in  its  action  upon  susceptible 
animals,   so   virulent,   indeed,   that  a   single  bacillus,   inoculated 


284  OTHER    GERM    DISEASES. 

under  the  skin,  may  be  sufficient  to  cause  the  disease  and  death. 
In  the  less  susceptible  animals  it  requires  a  larger  dose  to  produce 
similar  results.  The  lesser  susceptibihty  of  such  animals  as  the 
dog,  the  horse,  the  bird,  etc.,  renders  them  practically  immune 
against  spontaneous  infection,  and  the  disease  occurs  among  them 
only  as  the  result  of  artificial  experiments.  In  man  the  disease  is 
of  rare  occurrence,  being  practically  confined  to  people  dealing  in 
or  handling  hides  or  wool,  and  is  acquired  by  them  either  through 
abrasions  in  the  skin,  when  it  produces  malignant  pustule,  or  by 
breathing  the  spores  into  the  lungs,  when  it  is  called  wool-sorter^ s 
disease. 

Preventive  Inoculation. — Although  anthrax  is  an  extremely 
fatal  disease  to  animals  and  has,  in  the  past,  caused  heavy  losses 
to  agriculturists,  it  is  a  source  of  less  loss  to-day  than  in  former 
years,  since  it  can  be  fairly  well  controlled  by  preventive  inoculation. 
We  have  noticed  in  the  last  chapter  that  Pasteur  demonstrated  the 
important  principle  of  preventive  inoculation  by  his  experiments 
upon  anthrax;  the  discovery  has  been  of  great  practical  value. 
Cattle  can  be  protected  from  anthrax  by  inoculation,  and  from 
the  time  that  Pasteur  pointed  out  the  method,  hundreds  of  thousands 
of  animals  have  been  thus  inoculated  and  protected.  But  the 
protection  is  not  found  to  be  very  lasting,  and  animals  must  be 
inoculated  about  once  a  year  to  be  thoroughly  safe  from  the  disease. 
This,  of  course;  reduces  the  value  of  the  inoculation,  and  confines 
it  to  localities  where,  for  special  reasons,  the  disease  is  quite  common. 
It  also  explains  why  the  method  is  not  so  widely  in  use  now  as  at 
first;  but,  nevertheless,  large  amounts  of  the  inoculating  material 
have  been  used  in  this  country  as  well  as  elsewhere,  and  it  is  thought 
that  immense  losses  have  been  prevented  by  this  means  since  its 
discovery  by  Pasteur. 

OTHER  GERM  DISEASES  AMONG  ANIMALS. 

No  other  diseases  among  animals  have  acquired  so  much  interest 
as  tuberculosis  and  anthrax,  although  several  others  are  known  to 
be  produced  by  microorganisms  and  are  of  considerable  importance 


OTHER    GERM    DISEASES   AMONG   ANIMALS.  285 

to  agriculture.  Only  a  brief  mention  of  these  is  possible,  but  the 
following  list  includes  all  of  the  important  diseases  of  domesticated 
animals,  that  have  been  proved  to  be  caused  by  microscopic 
parasites. 

Swine  Plague,  Fowl  Cholera,  Rabbit  Septicemia,  Rinder- 
seuche,  Wildseuche  {B.  pleurosepticus). — These  names  are  ap- 
plied to  a  variety  of  affections  of  animals,  but  they  all  appear 
to  be  essentially  the  same  thing.  The  cause  is  a  bacterium  which 
was  first  identified  by  Pasteur  as  the  cause  of  fowl  cholera  and 
later  identified  as  the  inciting  agent  in  all  these  diseases.  They 
are  all  contagious  and  often  produce  considerable  havoc  among 
domestic  animals.  The  names  given  indicate  the  variety  of  animals 
attacked.  Rinderseuche  is  the  name  given  when  it  attacks  cattle, 
and  Wildseuche  when  it  attacks  deer;  septic  pleuropneumonia  and 
pneumoenteritis  are  also  names  applied  to  it. 

The  bacterium  causing  all  these  diseases  is  a  short  rod,  so  short 
as  sometimes  to  be  called  a  Micrococcus.  The  cultures  obtained 
from  different  animals  have  been  given  different  names,  B.  bovi- 
septicus,  B.  suisepticus,  etc.,  but  the  most  careful  study  fails  to 
show  differences  sufficient  to  warrant  their  separation,  and  the 
name  B.  pleurisepticus  has  been  suggested  as  indicating  its  relation 
to  its  many  hosts.  While  it  attacks  many  animals  it  is,  so  far  as 
known,  harmless  to  man.  It  produces  a  type  of  disease  quite  similar 
to  the  forms  of  blood-poisoning  which  have  been,  in  medical  practice, 
called  septicemia.  It  is  extremely  fatal  to  some  animals,  fowls 
and  rabbits  succumbing  to  its  action  with  extreme  rapidity  and 
with  almost  absolute  certainty.  Among  the  larger  animals  its 
course  is  not  necessarily  so  fatal,  but  in  all  those  referred  to  above 
the  disease  is  a  serious  one  and  almost  always  fatal.  When 
attacking  the  hog  it  produces  one  form  of  swine  plague,  this  being 
the  type  of  the  disease  most  commonly  found  among  domestic 
animals,  and  the  one  which  will  usually  be  most  interesting  to  the 
agriculturist. 

Hog  Cholera.  {B.  suipestifer.) — The  hog  cholera  is  a  disease 
related  to  the  last,  although  clearly  distinct  from  it,  and  is  one 
which  develops  spontaneously  in  swine  only.     It  is  quite  common 


286  OTHER    GERM    DISEASES. 

to  have  swine  plague  and  the  hog  cholera  together  in  the  same 
animal.  The  disease  sometimes  results  in  very  serious  losses. 
A  herd  of  swine  may  be  attacked  by  such  a  violent  epidemic  that 
90  per  cent,  of  the  animals  succumb  to  the  infection.  After  the 
death  of  the  animals  the  bacilli  which  produced  the  disease  are 
found  in  all  of  the  organs,  but  especially  in  the  spleen.  The  disease 
occurs  in  an  acute  form,  which  runs  its  course  with  excessive  rapidity, 
producing  death  in  twenty-four  hours,  and  in  chronic  form,  which 
has  a  slower  course,  lasting  from  two  to  four  weeks  before  finally 
resulting  in  the  death  of  the  animal.  The  organism  which  produces 
the  disease  is  named  B.  suipestifer  (or  B.  cholerce  suis),  and  is  very 
easily  cultivated  by  ordinary  methods  in  the  laboratory.  It  is 
capable  of  producing  the  disease,  not  only  in  the  swine,  but  in 
rabbits,  guinea-pigs,  mice,  and  some  other  animals;  but  as  a  spon- 
taneous affection  it  is  found  in  the  hog  only. 

Glanders.  Farcy.  Rotzbacillus  {B.  mallei). — This  disease, 
well  known  among  agriculturists,  occurs  not  infrequently  as  a 
normal  infection  in  the  horse  and  in  the  ass.  It  is  characterized 
by  the  appearance  of  ulcers  in  the  nasal  membranes,  by  enlarged 
submaxillary  lymphatics,  which  may  turn  into  open  discharging 
ulcers.  Later  the  lymphatics  of  the  whole  body  may  become 
tumor-like  swellings.  Other  parts  of  the  body  may  eventually 
be  affected.  The  secretions  from  the  various  ulcers  are  found  to  be 
decidedly  infectious,  and  it  is  through  these  ulcers  that  the  disease 
is  commonly  distributed.  It  occurs  in  an  acute  form  and  in  a 
chronic  form;  the  latter,  chiefly  in  the  skin,  receiving  the  name 
oi  farcy,  the  former,  chiefly  in  the  lungs  and  nasal  passages,  more 
commonly  known  as  glanders.  It  occurs  spontaneously  only  in 
horses  and  asses,  and  causes  great  losses  in  nearly  all  localities.  It 
may  occur  by  accidental  or  artificial  infection  in  many  other  animals. 
It  occurs  occasionally  in  men  who  have  become  accidentally  inoc- 
ulated in  the  treatment  of  horses  suffering  from  the  disease,  and 
when  it  does  occur  in  man  it  is  an  extremely  fatal  disease,  almost 
always  resulting  in  death. 

The  bacillus  which  produces  the  disease  is  named  B.  mallei. 
It  is  a  short  stationary  rod  which  lends  itself  readily  to  bacteriological 


OTHER   GERM   DISEASES   AMONG   ANIMALS.  287 

experiments.  It  is  found  to  be  capable  of  producing  the  disease 
in  cats,  dogs,  rats,  field  mice,  and  quite  a  variety  of  animals.  It  is 
only  slightly  pathogenic  for  the  sheep  and  the  mice.  The  pig  and 
the  cow  seem  to  be  immune  from  its  action. 

Symptomatic  Anthrax.  Black-leg.  Quarter-evil.  Rausch- 
brand  {B,  anthracis  symptomatici) . — This  disease,  with  its  variety 
of  names,  is  extremely  common  in  Europe.  It  has  been  rare  in  the 
United  States,  but  in  recent  years  is  becoming  more  abundant, 
being  found  as  an  epidemic  in  certain  herds.  It  is  a  disease  that 
occurs  chiefly  among  cattle,  and  is  characterized  by  certain  irregular 
swellings  in  the  subcutaneous  tissues  and  muscles.  The  swellings 
are  seen  especially  over  the  quarters  of  the  animal,  and  hence  the 
name  quarter-evil.  The  muscles  become  dark  colored  and  bloody 
(hence  the  name  black-leg),  and  contain  large  numbers  of  the 
bacilli  known  to  cause  the  disease.  It  is  the  cause  of  considerable 
trouble  to  raisers  of  cattle,  being  almost  universally  fatal,  although 
it  is  not  a  disease  that  can  be  regarded  as  extremely  common. 

The  organism  which  produces  the  disease  is  well  known  and  is 
named  B.  anthracis  symptomatici.  It  is  pathogenic  for  a  large 
number  of  animals  when  artificially  inoculated.  Swine,  dogs, 
rabbits,  fowls,  pigeons,  guinea-pigs,  and  horses  succumb  to  the 
disease  by  inoculation,  in  addition  to  cattle,  sheep,  and  goats,  in 
which  the  disease  occurs  spontaneously.  It  is  most  common 
among  cattle  as  a  spontaneous  affection,  and  quite  rarely  occurs  in 
sheep  and  goats.  In  the  horse  it  is  never  known  to  occur  spon- 
taneously. So  far  as  known,  the  bacillus  is  not  pathogenic  for  man, 
although  this  has  never  been  demonstrated;  but  no  instance  has  ever 
been  known  of  man  suffering  from  the  infection,  even  though  every 
opportunity  for  such  infection  has  been  offered.  The  disease  is, 
therefore,  not  regarded  as  injurious  to  man.  The  practice  of  in- 
oculating animals  against  the  disease  by  a  "preventive  culture"  is 
widely  and  successfully  adopted  in  the  United  States. 

Tetanus  or  Lockjaw  {B.  tetanus). — This  is  a  disease  of  rather 
rare  occurrence  among  domestic  animals,  but  it  may  sometimes 
occur  if  an  animal  receive  a  wound  by  means  of  some  object  that  has 
been  lying  for  a  long  time  in  the  soil.     The  cause  of  tetanus  is  a 


288  OTHER    GERM    DISEASES. 

bacillus  (B.  tetanus),  which  lives  normally  in  the  earth  and  may  get 
into  a  wound  and  produce  the  well-known  and  commonly  fatal 
disease. 

Abortion. — This  troublesome  disease  sometimes  appears  in 
a  herd  and  produces  great  loss,  and  endless  trouble  to  the  dairyman. 
Cows  attacked  by  the  disease  do  not  carry  their  calves  the  full  time, 
but  drop  them  early  and  become  useless  for  the  time  as  milk-cows. 
If  the  animal  is  once  affected  she  is  likely  to  have  the  same  trouble 
the  next  time  she  is  in  calf,  and  perhaps  her  usefulness  is  ended. 
This  trouble  has  for  some  time  been  recognized  as  contagious  and 
has  in  recent  years  been  demonstrated  to  be  produced  by  a  definite 
species  of  bacterium.  The  bacterium  may  infect  cow  after  cow,  and 
even  the  bull  may  distribute  it  through  a  herd  of  cattle.  The  best 
remedy  has  been  found  to  be  thorough  disinfection.  The  calf  must 
be  destroyed,  the  stable  disinfected,  genital  parts  of  the  cow  thor- 
oughly washed  with  disinfecting  solutions  and  the  animal  kept  from 
the  rest  of  the  herd.  A  thorough  disinfection  of  this  sort  will  com- 
monly allay  the  trouble. 

Takosis  of  Goats. — This  is  a  disease  of  goats  only  recently 
studied  and  found  to  be  caused  by  a  bacterium.  It  brings  on  a 
general  weakness  and  wasting  away,  which  finally  results  in  death. 
It  has  caused  great  loss  among  the  Angora  goats  in  the  northern 
states.     It  is  always  fatal. 

Lumpy  Jaw,  Malignant  Tumor,  Wooden  Tongue. — These 
three  names  are  applied  to  the  same  disease,  located,  however,  in 
different  places.  The  cause  of  the  trouble  is  one  of  the  higher 
types  of  fungi  rather  than  a  bacterium.  The  name  of  the  organism 
is  Actinomycosis  and  it  differs  from  bacteria  in  forming  longer 
threads  and  in  branching  (see  page  12).  When  it  finds  entrance 
into  cattle,  generally  through  the  mouth,  it  may  invade  the  tissues 
and  produce  the  diseases  named  above.  Sometimes  it  is  found 
in  the  throat,  lungs,  and  skin.  It  is  most  common  in  cattle,  but  it 
also  occurs  in  swine,  and  may  be  given  to  other  animals  by  inocula- 
tion. It  occasionally  occurs  in  man.  The  disease  is  not  common, 
though  it  causes  considerable  loss  at  times,  since  it  is  serious  and  apt 
to  be  fatal.     It  produces  hard  tumors  that  invade  the  bones  or  other 


OTHER   GERM   DISEASES  AMONG   ANIMALS.  289 

tissues,  causing  great  distortion.  The  disease  is  not  contagious  and 
its  source  is  as  yet  unknown. 

General  Inflammatory  Troubles. — Inflammatory,  suppura- 
tive, and  tumor-forming  troubles  are  liable  to  occur  in  almost  any 
part  of  the  body  of  man  or  animal.  These  are  commonly  caused  by 
bacteria,  particularly  by  the  class  called  Streptococci.  The  affections 
do  not  form  any  specific  disease,  but  receive  a  variety  of  names 
according  to  the  location  of  the  trouble.  For  example,  when  the 
streptococcus  produces  inflammation  of  the  udder  it  is  called 
garget,  mammitis,  or  mastitis,  while  hoof  rot  and  navel  ill  represent 
other  types  of  inflammation  located  elsewhere.  The  streptococci 
that  cause  garget  have  been  found  abundantly  in  the  milk  of  cows 
and  are  believed  to  be  the  reason  for  some  of  the  illnesses  in  man- 
kind that  follow  the  drinking  of  raw  milk.  Various  forms  of  sores, 
boils,  abscesses,  and  the  inflammations  following  wounds  are  also 
caused,  largely  or  wholly  by  streptococci,  and  most  types  of  inflamed 
tissue  in  an  animal  may  be  rightly  attributed  to  the  action  of  this 
class  of  bacteria. 

Another  bacillus  associated  with  a  variety  of  troubles  among 
animals  is  named  B.  necrophorus.  This  organism  produces  more 
than  an  inflammation;  it  gives  rise  to  a  general  decay  of  the  tissues 
attacked  (called  necrosis)  and,  since  it  attacks  many  parts,  it  has  a 
variety  of  effects.  In  the  skin  it  causes  numerous  inflammatory 
diseases.  It  produces  the  foot  rot  of  sheep  and  also  of  cattle.  It 
attacks  the  bones  in  the  nose,  causing  their  destruction;  it  may  bring 
about  troubles  in  the  alimentary  canal,  and  it  is  the  source  of  some 
of  the  cases  of  hog  cholera,  as  well  as  several  other  affections. 

Foul  Brood  of  Bees  (B.  alvei  and  B.  larvce). — Foul  brood  is 
a  disease  attacking  the  larvae  of  bees  while  still  within  their  cells 
causing  them  to  become  sickly  and  eventually  killing  them  and  pro- 
ducing a  decomposition  of  the  body.  The  hive  becomes  vile- 
smelling  from  the  decomposition  and  the  whole  economy  of  the  hive 
is  interrupted.  The  bees  fail  to  collect  honey  and  the  hive  may  be 
ruined.  There  are  really  two  different  diseases  going  by  this  name, 
the  American  and  the  European  foul  brood,  resembling  each  other 
and  yet  being  easily  distinguished.  Both  are  produced  by  bacteria, 
25 


290  OTHER    GERM    DISEASES. 

the  European  by  B.  alvei  and  the  American  by  B.  larvce.  In  this 
country  the  latter  is  the  more  common,  though  both  are  found. 
Both  diseases  are  readily  carried  from  hive  to  hive.  Sometimes  this 
is  done  by  robber  bees  that  steal  honey  from  hives,  and  sometimes  it 
is  carried  by  the  bee-keeper  who  handles  a  diseased  colony  and  then 
a  clean  colony,  or  who  places  in  a  clean  hive  honey  or  combs  from  an 
infested  hive.  Its  very  infectious  nature  should  be  thoroughly  ap- 
preciated by  the  bee-keeper  and  great  care  should  be  taken  in 
handling  bees.  It  is  also  doubtless  carried  from  locality  to  locality 
by  the  custom  of  selling  bees.  The  two  diseases  are  widespread 
over  America,  Europe,  Africa,  and  Australia.  It  spreads  rapidly, 
sometimes  infesting  a  whole  district  in  the  course  of  a  single  season 
so  as  nearly  to  ruin  the  industry  of  the  bee-keeper. 

DISEASE  CAUSED  BY  UNKNOWN  PARASITES. 

The  causes  of  several  well-known  diseases  have  not  yet  been  dis- 
covered; nevertheless  it  must  be  recognized  that  they  are  caused  by 
microorganisms  too  small  to  be  seen  by  our  microscopes.  That  they 
are  caused  by  living  agents  of  extremely  minute  size  is  shown  by  two 
series  of  facts :  i .  Material  may  be  obtained  from  animals  suffering 
from  the  disease  which  will  produce  the  disease  in  others,  but  its  power 
is  destroyed  by  the  same  disinfectants  as  those  used  to  kill  bacteria. 
2.  The  infectious  agent  will  pass  through  porcelain  filters,  whose 
pores  are  too  small  to  permit  even  the  smallest  bacteria  to  pass, 
while  it  will  not  pass  through  some  of  the  very  fine  porcelain  filters 
with  pores  still  smaller,  but  large  enough  to  allow  liquid  to  pass 
through  them.  There  are  other  reasons  for  the  conclusion,  but 
they  cannot  be  given  here.  Although  these  organisms  have  never 
been  seen,  quite  a  little  is  known  of  their  general  nature.  The 
animal  diseases  produced  by  invisible  organisms  are  the  following. 

Foot-and-mouth  Disease. — This  disease,  manifesting  itself 
chiefly  in  the  mouth  and  feet  of  cattle,  varies  much  in  its  severity. 
Although  not  often  causing  death,  it  does  result  in  great  financial 
losses  to  dairymen.  It  is  readily  transferred  to  other  animals, 
most  kinds  being  susceptible  to  it.     It  occurs  rarely  in  man,  being 


DISEASES   CAUSED    BY    UNKNOWN   PARASITES.  29 1 

transported  through  the  milk  of  diseased  animals,  but  it  is  not  a 
very  serious  matter,  being  a  mild  infection  only.  For  many  years 
it  has  caused  heavy  losses  in  the  cattle-raising  communities  of 
Europe.  It  has  not  been  common  in  the  United  States,  though  a 
few  cases  have  occurred  at  intervals,  and  there  have  been  two 
rather  severe  epidemics  within  the  last  ten  years.  These  epidemics 
have  been  vigorously  handled  by  the  agricultural  department 
and  have  been  speedily  stamped  out.  It  is  hoped  that  by  the  vigor- 
ous measures  taken  in  killing  all  cattle  attacked,  the  disease  may 
be  prevented  from  gaining  a  foothold  in  the  country,  and  that  our 
dairymen  may  thus  be  protected  from  the  troubles  and  losses  experi- 
enced elsewhere.  Hitherto  the  efforts  have  been  successful.  No 
other  remedy  is  known  save  that  of  isolation  or  slaughter. 

Rinderpest.  Cattle  Plague. — This  is  a  very  serious  disease, 
originally  found  in  Asia,  but  for  centuries  periodically  invading 
Europe,  and  recently  very  rife  in  Africa.  It  attacks  cattle  chiefly, 
and  its  death  rate  is  very  high.     Man  is  immune  against  it. 

Rabies,  or  hydrophobia,  is  also  produced  by  some  agent  not 
yet  surely  known.  It  attacks  dogs  chiefly,  although  occasionally 
it  is  found  in  horses,  cattle,  and  man.  So  far  as  known  the  only 
source  of  the  disease  is  the  bite  of  infected  animals,  and  the  great 
majority  of  cases  come  from  the  bite  of  dogs.  The  name  hydro- 
phobia applies  to  the  disease  in  man  only,  where  a  dread  of  water  is 
one  of  the  symptoms.  Since  this  dread  of  water  does  not  appear 
in  dogs  suffering  from  the  disease,  the  name  rabies  is  best  applied. 

Other  diseases  of  the  same  category  are:  pleuropneumonia, 
a  serious  disease  of  cattle;  horse  sickness,  a  destructive  disease  of 
horses  in  South  Africa;  bird  pest  (Vogelpest)  a  highly  infectious 
and  fatal  disease  of  chickens;  sheep-pox,  a  disease  of  sheep  in  the 
Mediterranean  countries.  In  all  of  these  the  exciting  agent  is 
unknown  and  is  probably  too  minute  to  be  seen  with  the  microscope 

ANIMAL  PARASITES. 

There  are  some  diseases  of  animals  caused  by  microscopic  animal 
parasites.     Of  these  the  only  well-known  example  in  this  country 


292  OTHER    GERM    DISEASES. 

is  Texas  fever  or,  as  it  is  frequently  called,  the  tick  fever.  This 
latter  name  is  given  to  it  from  the  fact  that  the  disease  is  dis- 
tributed by  means  of  the  cattle  tick.  Other  diseases  caused  by 
animal  parasites  are  surra  and  the  tsetse  fly  disease  or  nagana, 
neither  of  which  is  found  in  this  country. 

Diseases  of  Other  Animals. — ^Parasitic  diseases  are  found  in 
all  animals  bred  upon  the  farm,  each  animal  having  its  own  peculiar 
types.  Ordinary  fowls  have  fowl  cholera,  roup,  diarrheal  diseases, 
etc.,  and  turkeys,  geese,  and  ducks  have  diseases  of  their  own. 
Even  fishes  are  subject  to  diseases  produced  by  bacteria  of  other 
fungi.  It  is  impossible  in  this  work  to  consider  this  subject  further, 
but  it  is  well  to  bear  in  mind  that  the  list  of  parasitic  diseases  is  a 
long  one  and  is  being  increased  constantly  as  investigation  is  being 
extended. 


CHAPTER  XX. 
THE  PARASITIC  DISEASES  OF  PLANTS. 

It  is  by  no  means  easy  to  draw  a  sharp  line  between  plant  disease 
and  the  phenomenon  of  decay.  If  the  tissues  of  a  living  plant  show 
signs  of  decay  it  is  called  a  disease;  if  the  decay  occurs  in  fruit  or 
vegetables  after  they  are  harvested  we  speak  of  it  as  decay.  But 
there  are  some  parasitic  organisms  that  may  grow  in  the  living  plant 
and  thus  find  access  to  the  fruit  so  that  the  fruit  will  decay  after 
harvesting.  Should  this  be  called  a  disease?  In  some  cases  the 
parasites  seem  to  do  no  injury  to  the  living  plant,  but  live  in  its  tissues 
to  injure  the  stored  fruit  or  vegetable  later.  In  such  cases  it  is 
manifestly  difficult  to  say  whether  the  phenomenon  should  be 
called  a  disease.  In  the  types  given  in  the  following  pages  the 
parasites  in  most  cases  do  injury  to  the  living  tissues,  although  one 
or  two  are  exceptions. 

While  most  parasitic  diseases  of  animals  are  due  to  bacteria, 
with  a  considerable  number  caused  by  animal  parasites  and 
almost  none  by  the  higher  fungi,  a  different  condition  of  things 
is  found  among  plants.  The  larger  majority  of  plant  diseases  are 
caused  by  the  higher  fungi,  a  considerable  number  by  bacteria, 
while,  so  far  as  known,  none  are  caused  by  microscopic  animal 
life.  In  our  brief  survey  of  this  important  field  we  may  best  divide 
the  subject  into  two  divisions:  i.  The  Fungoid  Diseases.  2. 
The  Bacterial  Diseases. 

THE  FUNGOID  DISEASES  OF  PLANTS. 

This  is  by  far  the  largest  class  of  plant  diseases,  but  they  can  only 
be  touched  upon  in  this  work.  The  Fungi  that  cause  this  class  of 
diseases  are  mostly  of  some  size  and  can  hardly  be  called  micro- 
organisms.    They  do  not  therefore  strictly  belong  to  a  discussion  of 

293 


294  THE    PARASITIC    DISEASES    OF    PLANTS. 

bacteriology,  but  their  very  close  relation  to  germ  life  makes  it 
necessary  to  consider  them  briefly. 

As  stated  on  an  earlier  page,  the  higher  fungi  are  characterized 
by  developing  a  mycelium.  This  delicate  branching,  usually  color- 
less thread,  grows  in  profusion  in  or  upon  the  substance  that  furnishes 
the  fungus  with  its  nourishment.  It  is  this  mycelium  that  makes 
these  plants  especially  adapted  to  live  as  parasites  upon  plants.  A 
spore  of  some  fungus  falls  upon  the  surface  of  a  leaf  and  germinates, 
sending  out  its  tiny  thread.  This  finds  some  opening  into  which  it 
can  thrust  its  way.  Sometimes  the  opening  is  a  wound  in  the 
cuticle,  but  in  other  cases  it  is  the  breathing  pore  of  the  plant,  the 
stomata.  Once  it  has  entered  through  this  cuticle  it  finds  the 
tissues  soft  and  moist  and  there  is  nothing  to  prevent  its  growing 
through  the  plant.  The  mycelium  can  readily  grow  among  the 
plant  cells,  winding  its  way  in  all  directions  and  may  in  time  pene- 
trate to  all  parts  of  the  plant.  Living  thus  as  a  parasite  and  drawing 
its  sustenence  from  its  host  it  naturally  produces  more  or  less  effect 
upon  the  plant  life,  resulting  in  what  are  called  plant  diseases. 

The  mycelium  is  the  growing  part  of  the  plant,  but  not  its  repro- 
ducing part.  These  plants  are  reproduced  by  spores.  Although 
differing  in  their  method  of  origin,  the  spores  are  always  minute, 
microscopic  bodies,  produced  in  immense  numbers  by  the  fungus. 
Generally,  though  the  fungus  grows  below  the  surface  of  its  host, 
the  spores  are  produced  on  its  surface.  The  mycelium  is  usually 
out  of  sight,  while  at  certain  spots  the  parasite  breaks  through  the 
cuticle  of  its  host  in  order  to  produce  spores  and  discharge  them  into 
the  air.  The  mycelium  is  white  or  colorless,  but  the  spores  show  a 
variety  of  colors.  It  is  evidently  by  these  superficial  spores  chiefly 
that  the  fungus  is  spread  from  plant  to  plant. 

When  one  of  these  spores  gets  carried  to  the  surface  of  another 
plant  it  must  first  germinate  before  it  can  do  any  injury.  In  order 
to  germinate  it  must  absorb  moisture,  a  fact  that  explains  the  great 
influence  of  the  weather  upon  the  fungoid  diseases.  In  moist 
weather  the  spores  find  plenty  of  moisture  upon  the  surface  of  the 
leaves,  while  in  dry  weather  the  necessary  moisture  is  lacking.  Once 
it  has  germinated  and  its  mycelium  has  entered  the  plant,  it  finds 


THE    FUNGOID    DISEASES    OF    PLANTS.  295 

plenty  of  moisture  within  so  that  it  is  no  longer  dependent  upon  the 
weather. 

The  effects  produced  by  these  fungi  growing  in  the  plant  tissues 
are  extremely  varied.  Any  part  of  the  plant  may  be  affected,  some 
diseases  showing  in  one  place  and  others  elsewhere.  The  leaf  may 
become  covered  with  spots  of  various  colors,  or  it  may  wilt,  or 
roll  up  or  drop  off.  Scabs  may  grow  on  the  plant  or  its  fruit,  or  the 
whole  may  show  signs  of  rotting.  Plant  diseases  have  received 
various  popular  names  that  are  loosely  applied  and  not  very  clear 
in  their  meaning.  The  more  common  descriptive  names  are  the 
following : 

Wilts  are  characterized  by  the  wilting  and  withering  of  the  plant. 

Rots  are  characterized  by  a  tendency  of  the  plant  tissue  to 
soften  and  decay. 

Smuts  show  a  mass  of  black  or  blackish  spores. 

Mildews  show  a  whitish,  powdery  growth  over  the  surface  of  the 
host. 

Rusts  show  spots  of  a  reddish  color,  due  to  reddish-yellow 
spores. 

Anthracnose  is  a  name  frequently  applied  to  diseases  causing 
spots  on  the  leaves  or  elsewhere. 

Blight  is  a  term  with  no  definite  meaning,  but  is  generally  ap- 
plied to  almost  anything  that  causes  a  general  wilting  and  destruc- 
tion of  the  plant. 

These  terms  are  all  in  a  measure  descriptive  terms  of  the  effects 
produced  by  the  parasites  on  the  host.  None  of  them  are  specific 
diseases,  but  all  are  produced  by  many  different  parasites  on  many 
different  hosts,  and  in  some  cases  the  same  parasite  may  produce 
different  types  of  disease  at  different  stages  of  its  life. 

Methods  of  Combating  Fungoid  Diseases. — There  are 
several  general  methods  by  which  these  diseases  may  be  kept  in 
check:  i.  By  the  selection  of  resistant  varieties  of  the  ycultivated 
plants.  Experience  has  shown  that  some  varieties  yield  readily  to 
the  parasites  while  others  are  highly  resistant.  A  careful  selection 
of  the  varieties,  guided  by  experience,  is  sometimes  of  value  in 
checking  disease.     2.  By  regulating  the  conditions  of  the  cultivated 


296  THE   PARASITIC    DISEASES    OF    PLANTS. 

plant  in  such  a  way  as  to  render  it  best  able  to  resist  the  disease. 
This  involves  the  matter  of  cultivation,  fertilizing,  controlling  weeds 
that  serve  as  hosts  of  the  parasite,  etc.,  and  demands  a  knowledge 
of  the  methods  and  seasons  for  the  spore  distribution  of  the  fungi, 
and  each  disease  has  to  be  studied  as  a  separate  problem.  3.  By 
the  use  oi  fungicides.  These  are  properly  selected  chemicals  that 
act  as  powerful  germicides  upon  the  fungi,  but  do  not  injure  the  host- 
plant.  They  are  mostly  applied  by  spraying,  and  the  spray  reaches 
the  surface  of  the  plants  only,  being,  therefore,  of  little  or  no  value 
after  the  mycelium  has  actually  entered  the  plant  tissue.  Hence 
to  be  useful  they  must  be  applied  at  just  the  right  time,  and  each 
disease  must  be  carefully  studied  as  to  its  time  of  sporing  in  order 
that  spraying  may  be  a  success.  A  majority  of  the  successful 
fungicides  contain  copper  that  seems  to  be  especially  efficient  upon 
this  class  of  fungi.  4.  With  some  of  these  diseases  a  rotation  of 
crops  is  efficient,  since  a  fungus  that  attacks  one  host  may  be  without 
influence  upon  another  species  of  plant.  5.  Clean  seed  selection;  i.e., 
selection  of  seed  free  from  disease. 

To  describe  the  various  fungoid  diseases  is  impossible  in  this 
work.  A  list  of  the  more  important  ones  is  given  below,  classified 
according  to  their  host  plant,  with  the  popular  name  and  the  name 
of  the  fungoid  parasite. 

Alfalfa.     Leaf  spot,  Pseudopeziza  Medicaginis  (Lib.)  Sacc. 

Apple.  Bitter  rot,  Glomerella  rufomaculans  (Berk.)  Sp.  and 
von  Schr.  Black  rot,  Sphceropsis  malorum  Pk.  Canker,  Nectria 
ditissima  Tul.  Leaf  spots,  Phyllosticta.  Powdery  mildew,  Podo- 
sphcera  leucotricha  (Ell.  and  Ev.)  Salm.  Rust,  Gymnosporangium 
macropus  Lk.  Scab,  Venturia  incequalis  (Cke.)  Aderh.  Sooty 
blotch,  Phyllachora  pomigena  (Schw.)  Sacc. 

Asparagus.  Anthracnose.  Fusarium,  Rust,  Puccinia  asparagi 
DC.     Rust,     Cladosporium  herbarium  Link. 

Beans.  Anthracnose,  Colletotrichum  lindemuthianum  (Sacc,  and 
Magn.)  Bri.  and  Cav.  Downy  mildew,  Phytophthora  Phaseoli 
Thax.  Leaf  blotch,  Isariopsis  griseola  Sacc.  Rust,  Uromyces 
appendiculatus  (Pers.)  Lk. 

Beet,     Leaf  blight,  Cercospora  heticola  Sacc. 


THE   FUNGOID    DISEASES    OF   PLANTS.  297 

Cabbage.  Club  root,  Plasmodiophora  Brassicce  Wor.  Leaf 
molds,  AUernaria  BrassiccB  and  A.  macrospora  Sacc. 

Celery.     Leaf  blight,  Cercospora  Apii  Fr. 

Corn.  Leaf  blight,  Helminthosporium  turcicum  Pass.  Rust, 
Puccinia  Sorghi  Schw.     Smut,  Ustilago  ZecB  (Beckm.)  Ung. 

Cucumber.  Downy  mildew,  Plasmopara  cuhensis  (B.  and  C). 
Anthracnose,  Colletotrichum  lagenarium  (Pass) .  Scab,  Cladosporium 
cucumerinum  (Ell.  and  Arth.). 

Grape.  Anthracnose,  Sphaceloma  ampelium  DeBy.  Black 
rot,  Guignardia  hidwellii  (Via.  and  Rav.).  Downy  mildew, 
Plasniopora  viticola  (Ber.  and  De  Ton.)  Gray  mold,  Botrytis. 
Powdery  mildew,  Uncinula  necator  Bur. 

Muskmelon.  Anthracnose,  Collototrichium  lagenarium  Ell. 
and  Hals.     Downy  mildew,  Plasmopora  cuhensis  Humph. 

Oat.  Black-stem  rust,  Puccinia  graminis.  Smut,  Ustilago 
Avence  Jens. 

Onion.  Black  mold,  Macrosporium  Porri  Ell.  Black  spot, 
Vermicularia  circinans  Berk.  Downy  mildew,  Peronospora  schlei- 
deni  Ung.     Smut,  Urocystis  Cepulce. 

Peach.  Brown  rot,  Sclerotina  frutigena  Schrt.  Leaf  blight, 
Cercosporella  Persica  Sacc.  Leaf  curl,  Exoascus  deformans  Fckl. 
Scab,  Cladosporium  carpophilum  Thm. 

Pear.  Black  mold,  Fumago  vagans  Pers.  Leaf  spot,  Spetoria 
piricola  Desm.     Rust,  Gymnosporangium  globosum  Farl. 

Plumb.  Black  knot,  Plowrightia  morhosa  Sacc.  Brown  rot, 
Sclerotina  frutigena  Schrt.  Powdery  mildew,  Podosphcera  oxya- 
canthcB  DeBy. 

Potato.  Blight,  Phytpphora  infestans  (one  of  the  most  destruct- 
ive of  our  plant  diseases).  Dry  rot,  Fusarium  oxysporum  Schl. 
Scab,  Oospora  scabies  Thaxt. 

Squash.  Anthracnose,  Colletotrichum  lagenarium  (Pass.)  Ell. 
and  Hals.  Black  mold,  Rhizorpus  nigricans  Ehr.  Metallic  mold, 
Choanephora  cucurhitarim  Tha. 

Strawberry.  Fruit  rot,  Botrytis  vulgaris  Fr.  Leaf  blotch, 
Ascochyta  Fragarice  Sacc.     Leaf  spot,  Sphearella  Fragarice  Sacc. 

Tobacco.    Frost  fungus,  Botryosporium  pulchrum  Cda. 


298  THE    PARASITIC    DISEASES    OF    PLANTS. 

The  above  list  contains  only  a  few  of  the  very  large  number  of 
known  fungoid  diseases  of  plants,  but  will  serve  the  purpose  of 
showing  their  variety.  The  fungi  that  produce  these  diseases  are 
by  no  means  closely  related  to  each  other.  The  higher  fungi  are 
divided  into  many  classes  and  the  disease-producing  parasites  are 
distributed  among  them  all.  For  these  distinctions,  however,  the 
student  must  be  referred  to  books  upon  botany. 

THE  BACTERIAL  DISEASES  OF  PLANTS. 

Only  within  recent  years  has  it  been  appreciated  that  bacteria 
are  important  agents  in  producing  plant  diseases.  Even  after 
their  agency  in  causing  diseases  in  animals  had  been  fully  recog- 
nized it  was  denied  that  they  could  produce  troubles  of  this  sort 
in  plants.  Up  to  very  recent  date,  it  was  claimed  that  it  was  an 
impossibility  for  bacteria  to  penetrate  plant  tissue  so  as  to  produce 
trouble.  Plant  cells  are  provided  with  hard  cell  walls  of  cellulose 
and  wood,  which  protect  the  living  protoplasm  within;  and,  since 
these  cells  form  the  bulk  of  the  plant  and  are  adherent  to  each 
other,  it  was  difficult  to  see  how  bacteria  could  penetrate  into  the 
plant  at  all.  The  mycelium  of  the  higher  fungi  can  do  this  readily 
since  it  can  thrust  itself  between  the  cells,  and  thus  grow  easily 
within  the  solid  tissues;  but  it  seemed  impossible  to  believe  that 
bacteria  could  penetrate  the  hard  tissues.  Within  recent  years, 
however,  it  has  been  demonstrated  that  this  is  possible  and  the 
last  ten  years  have  disclosed  many  bacterial  diseases  of  plants, 
until  to-day  we  know  of  more  bacterial  diseases  of  plants  than  of 
animals. 

The  Black  Rot  of  Cabbage  (Pseudomonas  campestris). — An 
illustration  will  best  show  the  general  course  of  such  a  disease  and  at 
the  same  time  indicate  how  conclusive  is  the  proof  of  the  agency  of 
bacteria.  For  this  purpose  will  be  chosen  the  black  rot  of  the 
cabbage,  cauliflower,  turnip,  and  several  other  members  of  the  family 
CrucifercB.  The  disease  appears  first,  as  a  rule,  upon  the  edges  of 
the  leaves,  as  brown  spots,  that  spread  down  the  leaves  following  the 
veins  to  the  midrib  and  petiole  and  finally  into  the  main  stem  of  the 


THE    BACTERIAL    DISEASES    OF    PLANTS. 


299 


plant.  It  then  travels  rapidly  through  the  whole  plant  causing  the 
leaves  to  wilt,  turn  yellow,  dry  up  and  become  thin  and  parchment  like. 
The  veins  in  the  leaves  and  stem  are  particularly  affected  and  turn 
black,  this  being  the  characteristic  feature  of  the  .disease  and  the 
source  of  the  name  black  rot  (Fig.  53).  Sometimes  the  veins  alone 
are  affected.  Sometimes  the  trouble  does  not  appear  in  the  grow- 
ing plant,  but  only  in  the  cabbage  after  storing,  extending  through 
them  rapidly  and  ruining  them. 

When  these  black  veins  are 
studied  with  the  microscope  they 
are  found  to  be  filled  with  bacteria 
and  it  is  easy  by  proper  methods  to 
remove  them  and  cultivate  them  in 
the  laboratory.  Pure  cultures  of  an 
organism  are  thus  obtained,  Pseud, 
campestris  (Fig.  53).  It  is  easy  to 
keep  this  growing  in  the  laboratory 
for  months  under  strict  observation. 
Having  thus  obtained  a  pure  culture 
it  can  be  demonstrated  at  any  time 
that  it  will  produce  the  disease.  It 
is  only  necessary  to  dip  the  tip  of 
a  needle  into  the  pure  culture  and 
then  prick  the  leaf  of  a  healthy  plant 
with  it.    This  inoculation  is  followed 

in  a  few  days  by  the  appearance  of  the  characteristic  symptoms  of 
the  disease,  starting  at  the  point  of  the  needle  and  travelling  down 
the  plant  in  the  usual  way.  By  proper  study  it  is  possible  to  show 
that  the  bacteria  multiply  in  the  plant  following,  the  vascular  bun- 
dles which  they  first  turn  black  and  then  destroy.  Since  these 
bundles  convey  the  water  to  the  plant  their  destruction  shuts  off  the 
usual  water-supply,  and  the  plant  wilts.  It  is  possible  at  any  time  to 
isolate  the  bacterium  from  these  diseased  plants  and  obtain  it  again 
in  pure  culture.  Cabbage  plants  pricked  with  sterilized  needles 
show  no  evil  result,  proving  that  it  is  the  inoculated  bacteria  that 
produce  the  disease.     Such  experiments  as  these,  repeated  many 


Fig.  53.— The  black  rot  of  cabbage. 
a,  a  bit  of  the  stem  showing^the 
blackened  fibrovascular  bundles;  6, 
cells,  highly  magnified,  showing  some 
filled  with  bacteria;  c,  the  bacteria. 


300  THE   PARASITIC    DISEASES    OF    PLANTS. 

times  by  different  experimenters,  leave  no  room  to  dou5t  that  the 
bacteria  are  the  cause  of  the  disease  in  question. 

The  method  by  which  the  bacteria  make  their  way  into  the  plant 
is  interesting.  We  have  learned  in  an  earlier  chapter  that  bacteria 
frequently  secrete  enzymes.  This  Pseud,  campestris  secretes  such  an 
enzyme  and  one  that  has  the  power  of  softening  and  dissolving  cellu- 
lose. As  the  bacteria  multiply  at  the  inoculated  point  they  secrete 
this  enzyme,  called  cytase,  which  at  once  softens  and  disintegrates  the 
walls  of  the  adjacent  plant  cells.  The  contents  of  the  cells  thus 
exposed  are  quickly  killed  by  the  action  of  the  bacteria,  a  toxin  being 
probably  secreted  by  them  for  the  purpose,  and  the  bacterium,  feed- 
ing upon  the  food  thus  furnished,  multiplies  further.  More  cytase 
is  produced,  dissolving  more  cell  walls,  and  the  disease  progresses 
as  the  bacteria  thus  enter  the  plants.  In  this  way  they  travel 
through  the  plant,  chiefly  in  the  vascular  bundles,  and  finally  may 
affect  the  plant  throughout.  The  cellulose-dissolving  enzyme  has 
been  found  to  be  secreted  not  only  when  the  bacterium  is  growing 
in  the  host,  but  also  in  the  laboratory  in  the  bacteriologist's 
test-tubes. 

The  bacteria  have  apparently  three  methods  of  entering  the  plant. 
Through  the  uninjured  cuticle  they  are  unable  to  enter,  nor  can  they 
enter  through  the  stomata  of  the  plant.  But  if  the  cuticle  be  broken 
by  a  wound  or  scratch,  no  matter  how  tiny,  the  broken  cuticle  will 
offer  an  entrance  to  the  germs.  Further,  there  are  on  the  edges  of 
the  leaves  minute  openings  called  water  pores.  Through  these  pores 
the  bacteria  also  can  enter.  Seemingly  they  can  also  enter  through 
the  roots,  especially  through  the  tips  of  the  rootlets  which  are  likely 
to  be  exposed  and  broken  during  transplanting. 

No  effectual  remedy  against  this  disease  has  yet  been  found. 
Its  method  of  distribution  from  field  to  field  is  not  well  known,  and 
hitherto  no  means  of  checking  it  after  it  has  made  its  appearance  in 
a  field  has  been  discovered.  Wet  weather,  which  is  best  for  the 
growth  of  the  cabbages  is,  unfortunately,  also  best  for  the  growth  of 
this  parasite.  That  it  lives  in  the  soil  from  year  to  year  seems 
proved,  and  hence  after  it  makes  its  appearance  in  a  field  it  is  likely 
to  recur  year  after  year.     Since  it  is  confined  to  members  of  the 


OTHER   BACTERIAL   DISEASES   OF   PLANTS.  3OI 

Cruciferae,  it  is  a  natural  'suggestion  that  a  change  of  crop  to  some 
kind  of  plant  not  in  this  family  should  be  made  when  any  particular 
plot  of  ground  becomes  infested  with  the  disease.  The  destruction  of 
all  weeds  of  the  mustard  family  in  the  vacinity  of  cabbage  plots  is 
also  to  be  recommended.. 

OTHER  BACTERIAL  DISEASES  OF  PLANTS. 

The  illustration  given  will  serve  to  show  the  kind  of  evidence 
that  is  sought  for  in  the  study  of  plant  diseases.  In  the  list  that 
follows,  demonstrative  evidence  has  been  obtained  in  practically  all 
cases,  so  that  all  of  the  diseases  in  this  list  may  be  accepted  as  caused 
by  bacteria.  The  list  given  is  a  long  one,  and  if  it  be  compared  wi'  h 
the  list  of  animal  diseases  given  in  the  last  chapter  it  will  be  seen  to 
surpass  that  list.  These  plant  diseases  have  not  the  importance  nor 
have  they  developed  the  interest  that  attains  to  some  of  the  animal 
diseases,  but  nevertheless  they  are  of  great  significance  in  farming 
operations,  sometimes  causing  very  large  losses.  As  in  the  case  of 
animal  diseases  the  bacteria  causing  them  are  not  confined  to  one 
plant  host.  The  black  rot,  for  example,  attacks  the  cabbage,  the 
cauliflower,  the  turnip,  kale,  Brussels  sprouts,  collards,  rutabagas, 
radish,  as  well  as  some  other  plants,  all  of  the  family  Cruciferce.  So, 
as  with  the  other  diseases,  the  same  parasite  may  attack  several 
hosts. 

Classification  of  Bacterial  Diseases. — ^Plant  diseases  are  less 
clearly  defined  and  classified  than  animal  diseases.  Popular  names 
have  been  applied  to  them  without  careful  discrimination  till  the 
popular  names  have  ceased  to  have  any  sharp  meaning.  The 
bacterial  diseases  may,  however,  be  fairly  well  divided  into  three 
types,  distinguished  by  the  kind  of  effect  they  have  upon  the  host. 
These  are:  i.  The  Wilts.  In  these  the  bacteria  attack  chiefly  the 
vascular  bundles,  either  destroying  their  cells  or  clogging  them. 
This  shuts  off  the  ordinary  water-supply  to  the  plants  and  causes 
them  to  wilt  and  wither.  2.  The  Bacterioses  and  Rots.  In  these 
diseases  the  bacteria  invade  the  tissues  generally,  not  being  confined  to 
the  bundles,  and  destroy  the  plant  cells  at  once.     They  may  cause  the 


302  THE    PARASITIC    DISEASES    OF    PLANTS. 

tissues  to  become  much  softened,  thus  producing  the  soft  rots,  or  they 
may  fail  to  cause  this  softening  but  injure  them  in  other  ways,  so  that 
the  plant  does  not  rot  but  becomes  filled  with  bacteria  and  the  tissues 
are  much  injured.  These  are  sometimes  called  Bacterioses.  3, 
The  Tumor  Diseases.  In  these  cases  the  bacteria  cause  the  forma- 
tion of  unusual  growths,  tubercles,  or  tumors,  on  the  various  parts  of 
the  plants. 

The  Wilts. — The  black  rot  of  cabbage  belongs  to  this  class  and 
is  really  not  a  "rot,"  but  a  wilt.  In  addition,  three  others  will  be 
briefly  described. 

Brown  Rot  of  Potato,  Egg  Plant  and  Tomato  {B.  solanacearum) . — 
Although  frequently  called  a  rot,  this  disease  is  really  a  wilt.  It  is  a 
widely  distributed  disease  of  the  potato,  especially  in  the  northern 
part  of  United  States  and  Canada.  The  leaves  of  the  attacked  plant 
first  wilt  and  shrivel  and  then  the  stem  turns  brown  or  black.  The 
affection  extends  down  the  vascular  bundles  and  may  reach  the 
tuber.  In  this  it  spreads  through  the  vascular  bundles,  causing  in 
time  a  destruction  of  the  potato  that  has  given  to  it  the  name  of 
brown  rot.  The  bundles  are  found  to  be  filled  with  bacteria  in 
great  numbers,  that  destroy  the  cell  walls,  finally  causing  the  com- 
plete disintegration  of  these  tissues.  The  isolation  of  the  bacterium 
is  easy  and  inoculation  experiments  show  that  it  is  capable  of  pro- 
ducing the  same  disease  in  various  members  of  the  potato  family. 
The  bacterium  appears  to  be  carried  from  plant  to  plant  by  insects, 
and  the  potato  beetle  is  an  important  agent  in  its  distribution. 

The  term  potato  rot  is  applied  to  any  form  of  disease  that  is 
followed  by  the  rotting  of  the  potato.  There  are  several  different 
parasites  that  produce  this  phenomenon,  some  of  them  belonging 
to  the  higher  fungoid  types.  It  is  thought  that  this  bacterial  disease 
is  the  cause  of  the  larger  part  of  the  rots  in  our  Northern  States.  A 
second  bacterial  rot  of  the  potato  is  caused  by  a  bacterium  named 
B.  solanisaprus  (Har.).  It  is  also  a  wilt  rather  than  a  rot,  as  we 
have  used  the  terms,  although  after  it  affects  the  tuber  itself  it  pro- 
duces a  general  decay  of  the  tissues.  It  is  common  in  Canada, 
where  it  was  first  described.  A  third  bacterial  potato  rot  is  caused 
by  B.  atrosepticus  (VanHall). 


OTHER   BACTERIAL    DISEASES    OF    PLANTS.  303 

It  is  important  to  not€  that  the  bacterium  that  causes  the  brown 
rot  of  the  potato  can  also  live  as  a  parasite  in  the  tobacco  plant  where 
it  produces  what  is  known  as  the  Granville  wilt. 

The  Wilt  Disease  of  the  Gourd  Family  (B.  tracheiphiluSy  Sm.). — 
This  bacterium  attacks  various  members  of  the  gourd  family,  being 
best  known  in  the  cucumbers,  muskmelons,  pumpkins,  and  squashes. 
The  bacterium  that  produces  it  will  grow  readily  in  laboratory 
media  and  invariably  produces  the  disease  when  it  is  inoculated  into 
healthy  plants.  It  causes  the  wilting  of  the  plant  by  clogging  up 
its  vascular  ducts.  The  bacteria  appear  to  be  distributed  by  in- 
sects which  inoculate  the  plant,  chiefly  on  the  leaves,  by  puncturing 
or  by  eating  holes  in  them.  The  cucumber  beetle  and  the  squash 
bug  are  especial  offenders  in  this  respect,  and  anything  that  will 
keep  these  insects  in  check  will  help  to  reduce  the  troubles  from  the 
disease. 

The  Corn.  Wilt  (Pseudomonas  stewarti). — This  disease  affects 
sweet  corn  in  the  early  summer.  The  leaves  wilt  without  apparent 
cause  and  the  plant  gradually  withers  and  dies,  at  times  in  four 
days  and  at  others  as  much  as  a  month  is  required.  Sometimes 
the  attacked  plants  will  recover.  Usually  the  leaves  are  affected  one 
after  another,  but  sometimes  the  whole  field  seems  to  be  attacked  at 
once.  If  the  stem  is  cut  lengthwise  the  vascular  bundles  will  appear 
as  yellow  streaks,  which  become  black  in  the  dead  stems.  If  cut 
across,  these  bundles  exude  a  yellow  viscid  substance  that  is  com- 
posed mostly  of  bacteria  that  are  the  agents  that  produce  the 
disease.  The  germs  are  thought  to  be  distributed  by  the  seeds  of 
diseased  plants,  and  no  remedy  has  been  suggested  except  to  select 
resistant  varieties  of  corn,  and  to  use  care  not  to  plant  seed  from 
infected  plants. 

While  this  bacterium  attacks  only  sweet  corn,  there  is  another 
species  that  injures  field  corn.  This  has  been  variously  named 
{B.  ZecB,  B.  cloacce).  It  causes  quite  a  different  type  of  trouble, 
producing  dark  purplish  discolorations  on  the  leaf  sheath,  giving  a 
yellow  coloration  to  the  plant  and  causing  the  ears  to  undergo  a 
moist  rot.     It  also  attacks  the  broom  corn. 

A  wilt  of  the  sugar-cane  is  produced  by  Pseud,  vasculans. 


304  THE   PARASITIC   DISEASES    OF   PLANTS. 

The  Bacterioses   and  Rots. — A    single    illustration    of  this 
type  must  suffice. 

The  Fire  Blight  of  the  pear,  quince,  apple,  etc.,  (B.  amylovorus.) — 
This  bacterium  attacks  various  members  of  the  apple  family  and 
a  number  of  other  plants  as  well.  The  disease  has  been  known 
for  over  a  century  and  almost  every  conceivable  explanation  has 
been  given  for  it.  That  it  is  caused  by  a  bacterium  has  been  finally 
demonstrated  by  the  isolation  of  the  organism  and  the  reproduction 
of  the  disease  by  inoculation  experiments.  In  the  form  known  as 
the  twig  gall,  the  first  indication  of  the  disease  is  commonly  seen  in  a 
browning  or  blackening  of  the  leaves  of  the  young  shoots,  which  soon 
die.  It  then  extends  into  the  stem  by  the  way  of  the  inner  bark,  causmg 
it  to  become  blackened.  The  whole  of  this  tissue  is  destroyed  by  the 
bacterium,  causing  a  girdling  of  the  tree.  Then  it  extends  down  the 
stem,  sometimes  going  at  the  rate  of  an  inch  a  day,  and  eventually 
causing  great  injury  or  complete  destruction.  It-  particularly 
attacks  the  stored  starch,  converting  it  into  a  gummy  substance. 
The  diseased  area  may  extend  for  a  distance  down  the  stem  causing 
a  patch  of  "canker",  and  if  checked  in  its  growth  by  the  onset  of 
winter,  it  remains  alive  in  the  stem  till  warm  weather,  when  it  once 
more  begins  its  work  of  destruction.  In  moist  weather  a  viscid 
mass  extends  from  the  canker  spots,  containing  bacteria.  .Little 
is  known  of  its  means  of  distribution,  although  it  is  thought  that 
it  may  find  entrance  through  the  flowers  and  be  carried  to  them  by 
bees.  It  may,  however,  enter  through  wounds  in  the  bark  elsewhere. 
The  only  feasible  method  of  fighting  it  of  any  value  is  to  cut  away  the 
diseased  parts  as  soon  as  the  trouble  appears,  great  care  being  taken 
to  be  thorough  in  the  pruning  and  to  cut  away  every  bit  of  diseased 
wood. 

Other  examples  of  this  type  of  bacterial  diseases  are  the  following. 

Bean  blight,  produced  by  Bact.  Phaseoli. 

Cotton  bacteriosis,  produced  by  Bact.  malveacarum. 

Walnut  bacteriosis,  produced  by  Pseud,  juglandis. 

Mulberry  blight,  produced  by  B.  cubonianus. 

Black  spot  of  plum,  produced  by  Pseud.  Pruni. 

Wakker^s  disease  of  hyacinth,  produced  by  Bact.  Hyacinthi. 


OTHER   BACTERIAL   DISEASES    OF   PLANTS. 


305 


Soft  rot  of  turnips,  etc.,  produced  by  B.  oleracecB. 

Soft  rot  of  carrots,  produced  by  B.  caratovorus. 

Soft  rot  of  sugar  beet,  produced  by  B.  tenthium. 

Soft  rot  of  stored  celery,  caused  by  Pseud,  fluorescens. 

Rot  of  iris,  produced  by  Pseud,  iridis. 

White  rot  of  turnip,  produced  by  Pseud,  destructans. 

Gummosis  of  beet,  produced  by  B.  Beta. 

Soft  rot  of  onions  is  also  caused  by  bacteria,  and  there  are  some 
other  diseases  less  well  known. 

The  Tubercular  or  Tumor  Diseases.  The  Olive  Knot  {B. 
savastanoi) .  This  disease,  first  studied  in  1886,  and  attributed 
upon  insufficient  proof  to  a  bacterial  origin, 
has  recently  been  demonstrated  to  be  a 
bacterial  disease.  The  bacillus  is  a  motile 
one  with  several  flagella  at  one  end  and 
grows  in  ordinary  culture  media  in  the 
laboratory.  Several  different  bacteria  have 
been  found  associated  with  this  disease,  but 
the  one  to  which  the  above  name  has  been 
given  is  its  cause,  as  shown  by  the  fact 
that  the  inoculation  of  olive  trees  with 
cultures  of  the  organism  is  invariably 
followed  by  the  appearance  of  the  charac- 
teristic symptoms  of  the  disease  at  the  point 
inoculated.  The  effect  of  the  bacillus  is  to 
stimulate  the  plants  to  unusual  growth.  The  various  tissues  of  the 
stem  multiply  more  profusely  than  common,  producing  a  swollen 
growth  on  the  stem  which  is  called  the  olive  knot  (Fig.  54).  This 
injures  the  trees  and  sometimes  kills  them.  The  organism,  so  far 
as  known,  enters-  the  plant  exclusively  through  wounds.  It  occurs 
in  the  various  olive-raising  countries  of  Europe  and  Africa,  and  also 
in  California. 

The  Crown  Gall  of  the  Peach  and  Other  Plants  (B.  tumifaciens.) — 

This    disease,    until  recently   attributed    to    a    different    class   of 

fungi,  has  now  been  proved  to  be  caused  by  a  bacterium.     In][the 

peach  it  commonly  produces  an  enlarged  growth  at  the  crown  of 

26 


Fig.  54.— The   black  knot 
of  the  olive. 


3o6 


THE    PARASITIC    DISEASES    Of    PLANTS. 


the  plant,  between  the  stem  and  the  root.  The  parasite  that 
causes  it  has  the  power  of  growing  upon  a  large  series  of  plants, 
producing  tumors  in  various  parts  of  the  plant  which  injure  it 
more  or  less,  according  to  the  extent  of  the  infection.  Among  the 
plants  that  may  be  infected  with  it  are  the  raspberry  (Fig.  55),  the 
daisy,  the  hop,  the  radish,  the  cabbage,  the  tobacco,  the  sugar  beet, 
the  grape,  the  tomato,  the  oleander,  the  apple,  and  some  others. 
It  is  unusual  for  a  parasite  to  have  such  a  long  list  of  possible  hosts, 

but  in  all  these  plants  it  has  been  demon- 
strated by  Smith  that  tubercles  will  be 
produced  by  the  inoculation  of  pure 
cultures  of  the  organism.  It  is  the  cause 
of  considerable  losses  to  horticulturalists. 
Root  Tubercles  of  Legumes. — These 
have  been  considered  in  a  different  con- 
nection (Chapter  VII),  but  they  are 
properly  classed  here.  They  are  cer- 
tainly caused  by  parasitic  bacteria, 
although  in  this  case  apparently  both  the 
parasite  and  the  host  are  benefited  by  the 
association,  a  condition  sometimes  called 
symbiosis  rather  than  parasitism. 

Remedies. — Remedies  for  the  bacterial 
diseases  are  not  as  yet  very  satisfactory. 
Spraying,  so  frequently  efficient  against 
fungoid  diseases,  is  of  no  value  here,  be- 
cause the  bacteria  are  always  within  the  tissues  of  the  plant  where 
the  spray  cannot  touch  them.  Hence  in  dealing  with  plant  diseases 
in  general  it  is  always  desirable  to  know  whether  they  are  fungoid 
or  bacterial,  since  in  the  latter  case  spraying  is  always  useless. 
Each  disease  has  to  be  met  by  devices  adapted  to  the  peculiar 
nature  of  the  disease,  and  no  general  principles  can  be  given  beyond 
that  already  pointed  out  on  page  295. 


Fig.  55. — The  crown  gall  on  the 
loot  of  the  raspberry. 


PART  VI. 

APPENDIX. 
CHAPTER  XXI. 

LABORATORY  WORK. 

Laboratory  work  should,  so  far  as  possible,  accompany  the  study 
of  the  text.  It  is  impossible  to  make  a  series  of  experiments  that  will 
closely  follow  the  order  of  topics  in  the  text,  since  a  large  amount  of  pre- 
liminary work  has  to  be  done  in  making  media  before  actual  study  of 
bacteria  begins.  The  order  of  experiments  below  given  will  be  found  a  con- 
venient one  to  follow,  but  may  be  modified  to  suit  convenience. 

APPARATUS  NEEDED. 

Steam  sterilizer. 

Autoclave. 

Hot  air  sterilizer — A  common  gas  oven  used  for  cooking  will  do. 

Stew-pan  for  cooking  media. 

Flasks. — Liter  and  half-liter. 

Test-tubes,   heavy — Board  of  Health  pattern. 

Petri  dishes — four  inches  in  diameter. 

Pipettes — I  c.c.  and  2  c.c.  Some  larger  ones  are  also  convenient. 
Each  of  the  smaller  ones  should  have  a  glass  tube  holder  (Fig.  56)  or  there 
should  be  a  metal  box  with  cover  to  hold  fifty  or  more  pipettes  at  once. 

Wire  baskets  to  hold  test-tubes. 

Test-tube  racks,  or  common  tumblers  with  a  little  cotton  in  the  bottom 
to  hold  test-tubes. 

Beakers. 

Evaporating  dishes. 

Fermentation  tubes. 

Burette  holder  with  four  burettes. 

Evaporating  dishes. 

Measuring  cylinders — i  liter  and  100  c.c. 

Counting  plate  or  counting  card. 

Platinum  wire  to  be  fused  into  glass  rods. 
.  Culture  oven  with  constant  temperature  of  98°. 

Bunsen  burners. 


3o8 


LABORATORY    WORK. 


Fig  56.— 
1  c.c.  pipettes 
inclosed  in 
glass  tubes  for 
sterilizing. 


Forceps — Common  and  Cornet  forceps. 
Microscope  with  i  /12  immersion  lens  and  plenty  of  slides 
and  cover-glasses. 


MATERIALS. 


Peptone  (Witte) 
Salt 

Beef  extract 
Gelatin— Gold  label 
Agar-agar 

Litmus,  dry  in  cubes 
Absorbent  cotton 
Fuchsin 

Dextrose,  lactose,  and 
saccharose 


Normal    NaOH    and  Normal 

HCl. 
Phenolphthalein 
Alcohol 

Corrosive  sublimate 
NaOH 
HCI 

Azolitmin 
Methylene  blue 
Common  cotton,  good  quality 


No.  I.  Washing  and  Sterilizing  Glassware. — All  glass- 
ware must  be  thoroughly  washed  in  hot  water  and  soap. 
New  glassware  should  first  be  treated  with  i  per  cent.  HCl. 
Used  glassware,  containing  remains  of  gelatin  or  agar,  should 
first  be  boiled  in  water  containing  powdered  soap  or  sal  soda. 
Follow  this  with  thorough  washing  in  hot  water,  rinsing  in 
cold  water  and  draining. 

To  begin  with,  the  student  may  wash  50  test-tubes,  2  one- 
liter  flasks,  one  dozen  Petri  dishes  and  several  i  c.c.  pipettes. 
After  drying,  place  the  pipettes  in  glass  holders  or  a  metal 
box,  plug  the  test-tubes  and  flasks  tightly  with  cotton  and 
place  these  -with  the  Petri  dishes  in  the  sterilizing  oven. 
Heat  for  one  hour  at  310°  F.  (155°  C.)  for  one  hour.  All 
glassware  should  be  subsequently  sterilized  in  the  same 
way  before  using. 

No.  2.  Preparation  of  Agar  Culture  Medium  and  Bouillon. 
— Measure  out  the  following: 

Water i  liter 

Liebig's  extract  of  beef 3  grams 

Common  salt 5  grams 

Peptone 10  grams 

a.  Divide  in  two  lots,  set  one-half  aside  (see  below); 
place  the  other  half  in  a  stew-pan  and  carefully  weigh  the 
dish  and  its  contents,  noting  the  combined  weight  for  future 
use.  Dissolve  the  mixture  at  about  i  50°  F.  (6o°C.).  Add 
6  grams  of  agar-agar  (1.2  percent.),  cut  into  small  pieces,  and 
dissolve  by  heat.  It  takes  considerable  heat  to  dissolve  the 
agar  and  may  require  boiling.  Boil  till  the  agar  is  com- 
pletely dissolved  and  then  replace  the  water  that  has  evaporated  by  adding 
cold  water  till  the  original  weight  has  been  restored. 


MATERIALS. 


309 


If  CI 


y/oNormal 
NaON 


h.  Adjust  the  acidity  to  1.5  per  cent,  as  follows: 

Measure  out  10  c.c.  of  the  mixture,  placing  it  in  an  evaporating  dish 
with  ten  times  its  bulk  of  water.  Bring  to  a  boil  and  add  a  few  drops  of 
phenolphthalein  solution.*  This  solution  is  colorless  so  long  as  it  is  acid, 
but  turns  red  when  alkahne.  The  agar  medium  remains  red  after  the 
addition  of  phenolphthalein,  showing  it  to  be  acid. 

Place  the  evaporating  dish  under  a 
burette  containing  i/io  normal  NaOH 
(one  part  normal  NaOH  diluted  with 
nine  parts  of  distilled  water)  (Fig.  57). 
Take  the  reading  on  the  burette.  Allow 
the  solution  to  fall  drop  by  drop  into  the 
evaporating  dish,  stirring  between  each 
drop.  As  long  as  the  material  remains 
acid  no  color  will  appear.  Continue 
adding  the  NaOH  until  a  faint  red  color 
appears  and  does  not  disappear  upon 
stirring.  When  this  point  is  reached 
the  contents  of  the  evaporating  dish 
are  neutral. 

Take  the  reading  upon  the  burette 
and  determine  the  difference  between 
the  first  and  second  reading.  The 
difference  gives  the  number  of  cubic 
centimeters  of  i/io  normal  NaOH 
needed  to  neutralize  10  c.c.  of  the  cul- 
ture medium.  Divide  by  10  to  de- 
termine the  amount  necessary  to  neu- 
tralize one  CO.,  and  multiply  by  990  to 
determine  the  amount  necessary  to 
neutralize  the  rest  of  the  agar  medium. 

Add  to  the  agar  medium  sufficient 
NaOH  to  neutralize  it.  Instead  of 
adding  i  /lo  normal,  add  normal  NaOH, 
using  of  course  only  i/io  as  many  • 
cubic  centimeters  as  the  above  calcula- 
tion shows  would  be  needed  of  i/io 
normal  NaOH.  This  will  bring  the 
whole  to  the  neutral  point. 

Add  normal  HCl  to  the  amount  of 
15  c.c.  per  liter.  In  this  case,  there  being  only  990  c.c.  left,  the  amount 
added  should  be  14.9  c.c.  This  makes  an  acidity  of  1.5  per  cent.,  the  best 
reaction  for  most  bacteria. 

c.  Boil  vigorously  for  fifteen  minutes  and  then  restore  the  original 
weight  by  adding  water.     Test  the  reaction  again  as  before  to  see  if  it  is 

♦  Eight  per  cent,  phenolphthalein  in  50  per  cent,  alcohol. 


\ 


Fig.  57 — Two  burettes  arranged  for 
neutralizing  culture  media. 


3IO 


LABORATORY   WORK. 


correct.  It  should  require  1.5  c.c.  of  i/io  normal  NaOH  to  neutralize 
the  acidity  in  10  c.c.  If  the  reaction  is  right,  cool  to  about  60°  F.  and  add 
the  white  of  an  egg  which  has  been  mixed  with  a  little  water,  adding  slowly 
with  stirring.  Heat  slowly  and  allow  to  simmer  until  the  egg  has  co- 
agulated and  the  liquid  is  clear.  This  may  take  half  an  hour  or  more. 
Add  water  to  bring  to  the  original  weight  again,  plus  the  weight  of  the 
added  egg,  and  then  filter  as  follows: 

d.  Place  a  considerable  quantity  of  absorbent  cotton  in  a  large  funnel 
(Fig.  58).  Over  the  top  of  a  second  funnel  above  the  first,  place  some  cheese- 
cloth.   Pour  the  hot  agar  medium  into  the  cheese-cloth.    It  will  run  through 


Fig.  58. — Showing  method  of  filtering  culture  media. 


rapidly ;  then  filter  through  the  cotton  to  be  caught  below  in  a  beaker  or 
flask.  It  should  be  transparent,  clear,  and  of  a  yellowish-brown  color.  If 
it  is  not  clear  a  second  boiling  with  a  second  egg  will  usually  clear  it. 

Place  10  c.c.  of  this  agar  medium  into  each  of  25  test-tubes,  replacing 
the  cotton  plug.  In  filling  these  tubes  use  either  a  10  c.c.  pipette  or  a 
funnel,  taking  care  not  to  allow  the  agar  to  touch  the  sides  of  the  tubes 
where  the  cotton  is  to  be  inserted. 

e.  Place  the  test-tubes,  together  with  the  flask  containing  the  remain- 
der of  the  medium  (tightly  plugged  with  cotton)  in  the  autoclave  and  steril- 
ize at  I  5  lbs.  for  30  minutes.  Cool,  and  after  the  pressure  is  down  to  zero, 
remove  from  the  autoclave.  While  still  hot  lay  the  test-tubes  down  on  a 
table  with  their  mouths  slightly  raised,  so  that  the  agar  will  form  a  slanting 


MATERIALS. 


311 


surface  (Fig.  59).  Allow  them  to  harden  in  this  position  and  set  aside  for 
future  use.  Keep  this  and  all  other  culture  media  in  a  cool  dark  place  until 
used. 

Bouillon. — To  make  bouillon  the  procedure  is  as  above,  except  that  agar 
is  not  added.     The  other  half  of  the  dissolved  mixture  of  water,  peptone. 


Fig.  59. — Showing  method  of  hardening  agar  slants. 

salt,  and  beef  extract,  in  a,  is  brought  to  the  acidity  of  1.5  per  cent,  in 
exactly  the  same  way  as  above  described,  and,  after  a  boiling  (the  egg 
white  is  not  necessary  here)  the  bouillon  is  filtered  through  filter-paper, 
placed  in  tubes,  and  sterilized  in  exactly  the  same  manner  as  agar  medium. 


P'iG.  60. — A  steam  sterilizer  (Fowler). 


If  it  is  not  convenient  to  use  the  autoclave,  either  of  these  may  be 
sterilized  by  discontinouus  heating  as  follows:  Place  the  tubes  and  flask 
in  a  steam  sterilizer  and  steam  for  one-half  hour  (Fig.  60).  Set  aside  for 
twenty-four  hours  and  then  steam  for  another  half-hour.     Set  aside  once 


312 


LABORATORY    WORK. 


more  for  twenty-four  hours  and  then  steam  a  third  time.      If   properly- 
sterilized  these  media  will  keep  indefinitely. 

No.  2.  Determination  of  the  Number  of  Bacteria  in  Water. — Melt  three 
tubes  of  agar  as  above  made  in  hot  water  and  cool  to  about  125°  (500C.) 
With  a  sterilized  pipette  place  a  cubic  centimeter  of  water  from  any  source 
in  each  of  three  Petri  dishes  (Fig.  61.),  and  pour  the  agar  from  one  of  the 
melted  tubes  into  the  Petri  dish.  Replace  the  cover  again  at  once  and  by 
gentle  agitation,  thoroughly  mix  the  agar  in  the  dish  with  the  water  first 
added.  Do  not  do  this  violently  enough  to  spill  or  throw  the  agar  up  on 
the  sides  of  the  plate.  After  mixing  set  on  a  level  table  to  harden  and 
then  place  in  the  culture  oven.  These  preparations  are  called  agar  plates. 
After  twenty-four  hours  the  plates  will  be  seen  to  be  dotted  over  with 
colonies,  each  supposed  to  come  from  a  single  bacterium.  Count  the 
number  of  colonies;  this  will  give  the  number  of  bacteria  in  the  original 
c.c.  of  water  that  have  been  able  to  grow  in  the  medium. 


Fig.  61. — A  petri  dish  for  making  "plates." 


No.  3.  Water  Blanks. — Place  in  a  considerable  number  of  test-tubes 
9  c.c.  of  water.  In  a  series  of  small  flasks  or  bottles  place  99  c.c.  of  water 
(i.e.,  add  100  c.c.  and  then  remove  i  c.c.)  After  plugging  tightly  with 
cotton,  place  in  the  autoclave  and  sterilize  at  1 5  pounds  for  one  hour.  It  is 
very  desirable  to  have  test-tubes  and  bottles  used  for  these  water  blanks 
with  a  mark  etched  upon  them  at  the  9  c.c.  and  99  c.c.  level.  When  they 
are  to  be  subsequently  used  for  dilution  care  should  be  taken  to  see  that 
they  are  filled,  exactly  to  the  mark,  since  evaporation  frequently  with- 
draws some  of  the  water,  and  unless  they  contain  the  exact  quantity,  errors 
will  be  introduced. 

No.  4.  Determination  of  the  Bacteria  in  Milk. — Milk  commonly  contains 
so  many  bacteria  that  it  must  be  diluted  before  the  bacteria  are  determined. 
The  amount  of  dilution  needed  will  vary  widely  with  the  age  of  the  milk. 
For  most  market  milk  a  dilution  of  1,000  will  serve.  Proceed  as  follows: 
Into  a  99  c.c.  water  blank  place  i  c.c.  of  the  milk  to  be  tested.  Shake 
vigorously  so  as  to  distribute  the  bacteria  uniformly.  With  a  second  pipette 
transfer  i  c.c.  of  this  mixture  to  a  9  c.c.  water  blank  and  again  thoroughly 
mix.  With  a  third  pipette  place  i  c.c.  of  this  in  a  Petri  dish  and  then  pour 
upon  it  the  contents  of  an  agar  tube.  Agitate  as  in  the  last  experiment, 
allow  to  harden  and  incubate  in  the  oven  for  twenty-four  hours.  Use  a 
counting  plate  in  counting  the  colonies.     If  they  are  too  numerous  to 


) 


MATERIALS. 


313 


count  directly,  count  the  number  in  one  of  the  areas  in  the  counting  plate 
and  multiply  by  the  number  of  such  areas  covered  by  the  whole  plate. 
This  will  give  the  number  of  colonies  on  the  plate,  and  this  multiplied  by 
1000,  will  give  the  number  in  the  original  milk.  There  may  be  any  where 
from  a  few  thousand  to  several  millions.  It  frequently  happens  that  the 
dilution  of  1000  is  insufficient,  giving  too  many  colonies  on  a  plate.  Higher 
dilutions  are  needed  in  such  instances,  but  whether  to  make  a  higher  dilu- 
tion can  only  be  determined  by  a  knowledge  of  the  age  and 
temperature  of  the  milk  and  by  experience. 

From  these  and  from  the  plates  of  the  following  two  ex- 
periments isolate  and  purify  cultures  as  directed  in  No.  7. 

No.  5.  Bacteria  in  the  Air. — Pour  the  contents  of  several 
tubes  of  agar  into  Petri  dishes,  replace  the  cover  and  allow 
to  harden.  Remove  the  cover  from  one  of  them  and  allow 
it  to  remain  open  in  the  laboratory  for  two  minutes.  Then 
replace  the  cover  and  place  in  the  incubating  oven.  Expose 
a  second  in  the  same  way  in  a  barn;  a  third  out  of  doors;  a 
fourth  in  a  barn  after  hay  has  been  thrown  down  in  front  of 
cattle.  Other  Petri  dishes  may  be  exposed  in  other  localf- 
ties.  After  twenty-four  hours'  incubation  count  the  num- 
ber of  colonies  and  compare  the  relative  number  of  bacteria 
in  the  air  at  the  different  places. 

No.  6.  Bacteria  on  the  Fingers. — Pour  a  Petri  dish  as 
above  directed  and  allow  to  harden.  Remove  the  cover  of 
one  and  touch  it  gently  with  the  fingers  in  several  places. 
The  hands  may  then  be  thoroughly  washed,  and  a  second 
Petri  dish  treated  in  the  same  way.  Incubate  for  twenty- 
four  hours  and  see  if  bacteria  colonies  have  grown  where  the 
fingers  touched  the  agar. 

No.  7.  Isolation  and  Purification  of  Bacteria. — Any  of 
the  plates  above  prepared  will  show  after  proper  incubation 
a  number  of  colonies.  Comparing  the  different  colonies 
noticeable  differences  will  be  seen  between  them.  These 
differences  in  the  colonies  commonly  indicate  different  kinds 
of  bacteria,  for  the  same  kind  of  bacteria  produce,  com- 
monly, the  same  kinds  of  colonies. 

Isolation. — Sterilize  a  platinum  needle  (Fig.  62),  in  a  flame  until  red-hot 
and  after  allowing  it  a  few  seconds  to  cool,  dip  the  tip  of  it  into  one  of  the 
colonies.  Transfer  the  bacteria  adherent  to  the  needle  to  one  of  the  agar 
slant  tubes  by  removing  the  plug  and  drawing  the  tip  of  the  wire  over 
the  surface  of  the  agar.  Sterilize  the  needle  again  before  laying  it  down. 
Label  the  tube  with  a  gum  label  telling  its  source.  In  a  note-book  make  a 
brief  record  of  the  kind  of  colony  from  which  it  was  obtained.  Allow  the 
inoculated  tube  to  grow,  either  in  the  incubating  oven  or  in  the  room,  until 
a  growth  appears  on  the  surface  of  the  agar.     This  is  an  agar  slant. 

Purification. — If  the  colony  thus  isolated  has  grown  from  a  single  bac- 
terium this  growth  on  the  agar  will  be  a  pure  culture.      But  this  is  not 
27 


Fig.  62.— 

P  latinum 

needle    and 

platinum  loop. 


314  LABORATORY    WORK. 

always  the  case  and  therefore  the  culture  must  be  purified.  With  the  tip 
of  a  platinum  needle  remove  a  minute  quantity  of  the  growth  on  the  agar 
and  place  it  in  a  9  c.c.  water  blank.  Thoroughly  shake  and  transfer  two 
platinum  loopfuls  of  the  water  to  a  melted  agar  tube.  Shake  gently  so 
as  to  mix,  but  not  to  produce  bubbles,  and  then  transfer  two  loopfuls  of 
this  agar  to  a  second  agar  tube,  mixing  as  before.  Pour  the  contents  of 
each  agar  tube  into  a  Petri  dish,  harden  and  incubate  as  usual.  If  the  cul- 
ture was  pure  the  colonies  should  be  all  alike.  Pick  out  one  of  them  with 
the  platinum  needle,  inoculate  it  upon  another  agar  slant  and  label  it  a 
pure  culture  from  milk  or  whatever  may  have  been  its  source.  In  this 
condition  it  may  be  set  aside  and  preserved  for  a  long  time.  As  long  as  the 
agar  remains  moist  the  bacteria  will  usually  be  alive. 

In  the  above  manner  isolate  and  purify  a  considerable  number  of  cul- 
tures of  bacteria  from  the  plates  made  in  Nos.  4-6,  and  keep  these  in  a  cool, 
aark  place  for  use  in  various  experiments  given  below. 

No.  8.  Microscopic  Study  of  Bacteria. — Prepare  one  or  both  of  the 
following  staining  solutions: 

Methylene   Blue. 

Saturated  alcoholic  solution  of  methylene  blue,    1 5  cm. 
Potassium  hydrate  (1:10,000),*  50  c.c. 

Fuchsin  Solution. 

Saturated  alcoholic  solution  of  fuchsin,  5  c.c. 

Five  per  cent,  solution  of  carbolic  acid,  45  c.c. 

In  the  middle  of  a  clean  microscopic  slide  place  a  drop  of  water  (steril- 
ized). With  a  platinum  needle  remove  a  very  small  quantity  of  the  bac- 
teria growth  from  the  surface  of  one  of  the  slant  cultures  prepared  in  No.  7 
and  place  in  the  drop  of  water.  Stir  this  drop  with  the  needle,  to  distribute 
the  bacteria  and  then  (a)  spread  it  over  the  slide.  Allow  it  to  dry  in  the  air, 
and  then  pass  the  slide  three  times  slowly  through  a  gas  flame.  The  pur- 
pose of  this  is  to  (6)  fix  the  bacteria  firmly  to  the  slide  so  that  they  will 
not  be  washed  away.  It  is  necessary  not  to  use  too  much  heat.  This  may 
also  be  done  by  leaving  the  slide  for  a  few  minutes  on  a  water-bath.  After 
fixing,  cover  the  bacteria  completely  with  several  drops  of  one  of  the  stain- 
ing solutions  and  allow  to  (c)  stain  for  several  minutes.  The  length  of  time 
necessary  for  this  varies  with  conditions,  one  to  five  minutes  being  usually 
sufficient.  Wash  off  the  stain  in  running  water  and  then  dry  the  slide  by 
gentle  heat.  Place  a  drop  of  immersion  oil  upon  the  stained  bacteria  and 
place  the  slide  under  the  microscope.  Use  a  i  /12  inch  immersion,  lowering 
the  objective  into  the  immersion  oil  and  focusing  very  carefully.  If  the 
microscope  has  an  Abbe  condenser  or  a  diaphragm  it  is  best  to  have  this 
widely  open.  The  bacteria  are  so  minute  that  it  is  hardly  possible  to  study 
them  with  a  lower  magnifying  power  than  a  1/12,  although  they  can  be 

*  To  make  this  solution  add  i  c.c.  of  a  10  per  cent.  KOH  solution  to  99  c.c.  of  water,  and  then 
add  5  c.c.  of  this  to  45  c.c.  of  water. 


MATERIALS.  315 

seen  with  a  i  /6  inch.  Examine  the  bacteria  and  sketch.  In  this  way 
make  a  microscopic  examination  of  all  of  the  cultures  isolated  and  puri- 
fied, and  compare  with  Figs.  7  and  9.  If  it  is  desired  to  preserve  the  speci- 
men place  a  drop  of  Canada  balsam  on  the  bacteria  after  drying,  and 
then  cover  with  a  cover-glass. 

Motility. — To  determine  the  motility  of  bacteria  transfer  a  small  quan- 
tity from  an  agar  slant  to  a  bouillon  tube,  and  allow  to  grow  for  24  hours. 
Place  a  drop  of  the  24-hour-old  bouillon  culture  on  a  slide  and  put  upon  it 
a  cover-glass.  Examine  this  with  a  i  /6  inch  objective  and  with  the  dia- 
phragm nearly  closed.  The  best  light  for  the  purpose  is  artificial  light  (elec- 
tric) placed  near  the  microscope  and  reflected  through  by  the  plain  surface 
of  the  mirror.  It  will  be  very  difficult  at  first  to  see  the  bacteria,  but  with 
careful  focusing  they  will  appear  as  transparent  dots  or  rods.  Examine 
carefully  to  determine  whether  they  are  stationary  or  motile,  calling 
only  those  motile  that  move  back  and  forth  across  the  stage  and  not  those 
that  simply  dance  back  and  forth  without  locomotion  (the  Brownian 
motion).  It  is  sometimes  desirable  to  keep  the  specimen  under  observa- 
tion for  some  time  in  which  case  a  hanging-drop  method  may  be  used. 
A  concave  slide  is  to  be  used  and  a  ring  of  vaseline  painted  around  the 
depression.  The  drop  containing  the  living  bacteria  is  placed  in  the 
middle  of  a  large  cover-glass  and  inverted  over  the  concavity  of  the  slide. 
By  pressing  it  firmly  into  the  vaseline  ring  it  will  be  sealed  so  as  to  prevent 
evaporation  and  may  be  kept  under  observation  for  hours. 

No.  9.  Bacteria  in  the  Mouth. — With  a  clean  knife  scrape  a  little  of 
the  material  attached  to  the  teeth  and  spread  it  in  a  very  thin  layer  over  a 
slide.  Dry,  fix  and  stain,  and  with  a  microscope  note  the  large  numbers 
of  bacteria  present.     Sketch  the  varieties  seen. 

No    10.  Gram  Stain. — Prepare  the  following: 

Anilin  Oil  Gentian  Violet. 

Saturated  alcoholic  solution  of  gentian  violet,       6  c.c. 
Absolute     alcohol,  5  c.c. 

Anilin  water,*  50  c.c. 

Grams  lodin  Solution. 

lodin,  I  gm. 

Potassium  iodid,  2  gm. 

Distilled  water,  300  c.c. 

Spread  and  fix  on  a  slide,  a  little  of  one  of  the  cultures  of  bacteria, 
and  stain  for  one  and  one-half  minutes  in  the  gentian  violet  solution.  Pour 
off  stain,  without  washing,  and  place  in  the  iodine  solution  for  one  and  one- 
half  minutes.  Apply  95  per  cent,  alcohol  until  the  drippings  do  not  stain 
white  filter-paper.  This  will  take  about  three  minutes  and  the  specimen 
will  be  largely  decolorized.     Wash  in  water  and  study  with  microscope  to 

♦Made  by  adding  2  to  3  c.c.  anilin  oil  to  50  c.c.  of  water  and  shaking  thoroughly,  with  subse- 
quent filtering. 


3l6  LABOEATORY    WORK. 

see  if  the  bacteria  are  still  stained  blue  or  have  been  decolorized.  Differ- 
ent species  differ  in  this  respect.  If  they  are  still  stained  they  are  Gram- 
positive;  if  decolorized,  they  are  Gram-negative. 

No.  II.  Microscopic  Study  of  Yeast. — Rub  up  a  bit  of  an  ordinary  yeast 
cake  in  a  little  water.  Place  a  dilute  drop  on  a  slide  and  proceed  to  stain 
exactly  as  above  described  for  bacteria.  Study  with  1/12  immersion  lens 
and  compare  with  yeast  as  to  size  and  shape.  Look  over  the  specimen  and 
find  some  cells  that  show  buds. 

No.  12.  Gelatin  Culture  Medium. — a.  Weigh  out  the  same  ingredients  as 
directed  in  No.  2,  omitting  the  agar.  After  the  mixture  has  dissolved  add  1 2 
per  cent,  of  first-grade  gelatin.  Allow  to  soak  till  soft  and  almost  melted. 
Weigh  the  dish  with  its  contents.  Bring  to  a  boil  slowly  and  boil  for  five 
minutes  and  add  water  to  restore  original  weight. 

b.  Determine  the  acidity  and  bring  the  reaction  to  1.5  as  described  in 
No.  2. 

c.  Boil  1 5  minutes.  Cool  and  add  the  white  of  an  egg  dissolved  in  a 
little  water.  Heat  slowly  to  boiling  and  allow  to  boil  gently  till  the  egg 
is  coagulated  and  the  liquid  clear ;  usually  about  1 5  minutes.  Replace  the 
evaporated  water. 

Heat  once  more  to  boiling  and  filter  through  absorbent  cotton.  Place 
10  c.c.  in  each  of  about  fifty  tubes  and  the  rest  in  a  flask,  plugging  both 
with  cotton.  Sterilize  in  the  steam  sterilizer  for  twenty  minutes.  Set 
aside  for  twenty-four  hours  and  steam  a  second  time;  this  time  allow 
half  an  hour  steaming.  Set  aside  for  another  twenty-four  hours  and  steam 
again.  Gelatin  cannot  be  sterilized  in  the  autoclave  satisfactorily,  since 
too  high  a  heat  will  prevent  its  subsequently  hardening  when  cooled. 
Too  long  boiling  will  in  the  same  way  ruin  the  gelatin. 

No.  13.  Litmus  Gelatin  and  Litmus  Agar. — These  are  used  chiefly  to 
detect  acid-producing  bacteria.  Make  agar  or  gelatin  in  the  manner  al- 
ready described,  except  that  i  per  cent.'  lactose  is  added,  and  1.5  per  cent, 
agar  instead  of  1.2  per  cent.,  or  15  per  cent,  gelatin  instead  of  12  per  cent. 
These  are  to  be  filtered  in  the  usual  way,  and  are  known  as  lactose  agar 
and  lactose  gelatin. 

Prepare  a  litmus  solution  by  soaking  50  grams  of  dry  litmus  cubes  with 
250  c.c.  water.  Soak  for  twenty-four  hours  and  filter  through  filter-paper. 
Add  enough  of  this  to  the  agar  or  the  gelatin  to  give  it  a  blue  color.  Tube 
the  medium  as  usual  and  sterilize. 

Instead  of  using  the  litmus  solution  azolitmin  may  be  used.  This 
is  a  powder  that  may  be  dissolved  in  a  little  alcohol  and  sufficient  added  to 
the  lactose  agar  or  lactose  gelatin  to  give  a  blue  color.  Its  use  is  simpler 
and  in  some  jespects  better  than  litmus  solution. 

No.  14.  Gelatin  Plates  from  Milk. — Procure  some  milk  that  is  not  more 
than  six  hours  old.  Dilute  1000  times,  as  directed  in  No.  4.  Place  i  /2  c.c. 
of  the  dilution  in  two  Petri  dishes  and  i  c.c.  in  two  others.  Pour  into  each 
the  melted  gelatin  from  one  of  the  gelatin  tubes  prepared  in  No.  12. 
Thoroughly  mix  by  gentle  agitation  and  place  in  a  cool  place  to  harden. 
Set    aside   at  a  temperature  of    about  70°  for  the  bacteria  to  grow.     If 


MATERIALS. 


317 


the  temperature  rises  above  80°  the  gelatin  is  likely  to  melt  and  spoil  the 
plate.  It  takes  about  two  days  for  the  colonies  to  appear.  After  two  or 
three  days  carefully  study  the  plate,  noting  the  liquefying  colonies  and  the 
non-liquefying  colonies.  Note  also  other  differences.  Isolate  and  inocu- 
late upon  agar  slants  several  of  the  different  colonies,  including  both 
liquefiers  and  non-liquefiers. 

Litmus  gelatin  or  litmus  agar  plates  should  be  made  in  the  same  way,  and 
the  appearance  of  a  red  color  around  some  of  the  colonies  will  make  it  pos- 
sible to  detect  the  acid-forming  bacteria. 

No.  15.  Potato  Tubes. — Select  a  large  fair  potato,  care-  ,    ^  ^ 

fully  wash  and  peal.  With  a  special  cutter  or  with  a  broken 
test-tube  with  sharp  edges,  bore  out  some  cylindrical  plugs 
of  potato.  Cut  them  obliquely  so  as  to  make  two  wedge- 
shaped  pieces  of  each  plug,  and  soak  in  running  water 
overnight.  In  the  bottom  of  some  large  test-tubes  place 
a  little  cotton  and  enough  water  to  cover  it  (Fig.  63).  Place 
a  single  potato  slant  in  each  tube  and  sterilize  by  one-half  || 

hour  steaming  upon  three  successive  days. 

No.  16.  Milk  Tubes. — Place  about  10  c.c.  of  skim  milk  in 
test-tubes  and  sterilize  for  one-half  hour  on  three  successive  ||||||l 

days.  The  milk  should  be  first  tested  with  litmus-paper, 
and  if  acid,  should  be  made  neutral  by  adding  NaOH.  ||{| 

Litmus  Milk. — This  is  made  as  above,  except  that  enough 
litmus  solution  is  added  before  sterilizing  to  give  a  moder- 
ately blue  color. 

No.  17.  Fermentation  Tubes. — Prepare  200  c.c.  of 
bouillon  as  described  in  No.  2,  adding  to  it  2  grams  lactose 
and  adjust  the  reaction  to  the  neutral  point.  To  a  second 
lot  of  bouillon  add  the  same  amount  of  dextrose  and  to  a 
third  lot  the  same  amount  of  saccharose.  After  dissolving 
and  filtering  pour  into  fermentation  tubes,  enough  to  fill  the 
closed  arm  and  half  the  bulb  (Fig.  64).  A  dozen  or  more 
tubes  of  each  of  these  bouillons  should  be  prepared  and  Fig.    63. — 

labeled.  Sterilize  by  steaming  on  three  days.  If  gas  col-  Potato  tube, 
lects  in  the  closed  arm  remove  by  tilting  the  tube. 

No.  18.  Testing  Characters  of  Bacteria. — Several  isolated  and  purified 
cultures  of  bacteria  have  been  prepared  in  No.  7.  After  having  prepared 
the  several  culture  media  above  described,  use  them  as  follows :  Make  a 
fresh  agaT  slant  from  each  purified  bacteria  culture  and  allow  to  grow 
about  24  hours.  If  possible  use  a  culture  of  B.  coli  for  one  of  the  series  of 
tests.     Then  inoculate  with  a  small  quantity  of  the  growth  the  following. 

a.  Two  agar  slants,  h.  One  gelatin  stab.  This  is  made  by  dipping  a 
straight  platinum  needle  into  the  bacteria  and  then  thrusting  it  straight 
into  the  gelatin  of  a  gelatin  tube,  in  the  middle  of  the  tube,  and  forcing  the 
needle  to  the  bottom,  carefully  withdrawing  without  disturbing  the  gelatin. 
c.  A  fermentation  tube  of  each  of  the  three  sugars,  d.  Two  milk  tubes. 
/.  Two  litmus  milk  tubes,     g.  Two  potato  tubes,  inoculating  the  potato'on 


3i8 


LABORATORY    WORK. 


its  surface  only.  Place  one  of  the  agar  tubes,  one  of  the  milk  tubes,  litmus 
milk  tubes,  and  potato  tubes  in  the  incubating  oven  at  98°.  Place  all  others 
at  room  temperature.  Allow  to  grow  several  days,  examining  each  day, 
For  each  species  of  bacterium  note  the  following  points: 

Agar  Slants;  Color.  Is  growth  thick  or  thin,  moist  or  dry;  does  it 
spread  ? 

Morphology. — With  the  microscope  study  stained  specimens;  note 
shape — formation  of  chains — spores.     Determine  motihty. 

Gelatin  Stab. — Note  liquefaction,  needle  growth,  surface  growth. 
Compare  growth  with  Fig.  35. 

Bouillon. — Note  turbidity,  scum,  sedi- 
ment. 

Milk  Tubes. — Note  at  98°  and  70°  the 
development  of  acid  as  shown  by  its  action 
on  litmus,  curdling,  separation  of  whey, 
appearance  of  gas  bubbles,  subsequent 
softening  and  solution  of  the  curd,  called 
digestion. 

Potatoes. — At  98°  and  70°,  note  color, 
abundance  of  growth,  texture  of  growth, 
discoloration  of  potato. 

Fermentation  Tubes. — After  several 
days  note  whether  gas  has  collected  in  the 
closed  arm.  If  not,  record  the  bacterium 
as  forming  no  gas.  Test  the  liquid  in  the 
bulb  with  litmus-paper  to  see  if  acid.  If 
gas  is  formed  in  any  tube  place  a  mark 
on  the  tube  at  the  top  of  the  liquid  to 
mark  the  amount  of  gas.  Then  fill  the 
bulb  completely  with  a  2  per  cent.  NaOH 
solution.  Place  the  thumb  over  the  open- 
ing of  the  bulb  so  that  there  will  be  no  gas 
bubble  between  the  thumb  and  the  surface  of  the  liquid.  Invert  the 
tube,  allowing  the  gas  to  flow  into  the  bulb,  and  by  turning  back  and 
forth  mix  the  gas  with  the  NaOH  solution,  keeping  the  thumb  in  position 
all  the  time.  After  mixing  turn  the  tube  once  more  so  that  all  the  gas 
will  be  in  the  closed  arm.  Remove  the  thumb  and  it  will  usually  be  found 
that  the  level  of  the  liquid  in  the  closed  arm  rises,  because  some  of  the  gas 
has  been  dissolved  in  the  NaOH.  This  dissolved  gas  is  CO2.  By  deter- 
mining the  level  of  the  gas  before  and  after  the  treatment  the  proportion 
of  CO2  to  the  undissolved  gas  may  be  determined.  This  is  called  the  gas 
ratio. 

Tabulate  the  characters  of  the  different  species  of  bacteria  tested, 
determining  whether  any  two  of  them  are  alike  in  all  respects.  It  is 
by  such  characters  that  bacteria  are  described.  To  determine  species  is 
too  difficult  for  a  beginner. 

No.  20.  Test  for  Indol. — Make  the  following : 


Fig.  64. — ^Fermentation  tube  with 
closed  arm  containing  gas. 


MATERIALS.   •  319 

Dunham's  Solution. 

Peptone,  i      gm. 

Sodium  chlorid,  o .  5  gm. 

Distilled  water,  100  c.c. 

Dissolve,  place  in  test-tubes  and  sterilize  as  usual.  After  sterilization 
uioculate  tubes  with  several  different  pure  cultures  of  bacteria  and  allow 
to-  grow  for  10  days.  Add  i  c.c.  of  a  0.0 1  per  cent,  solution  (fresh)  of 
potassium  nitrite  and  a  few  drops  of  concentrated  sulphuric  acid.  Heat 
gently.  If  a  pink  color  appears  it  indicates  the  formation  of  indol,  a  charac- 
ter used  to  distinguish  certain  species  of  bacteria  {e.g.,  B.  coli). 

No.  21.  Putrefa(;tion. — Place  in  a  series  of  test-tubes  with  a  little  cold 
water  the  following:  a.  A  bit  of  raw  meat;  b.  some  white  of  egg;  c. 
some  flour;  d.  some  crushed  beans;  e.  sugar;/,  starch;  g.  a  bit  of  melted 
butter.  Set  in  a  warm  place  for  two  or  three  days  and  determine  which 
will  putrefy  and  which  will  not. 

From  the  tube  containing  the  meat  and  the  egg,  remove  a  bit  of  the  liq- 
uid as  soon  as  putrefaction  begins  and  examine  under  a  miscrocope  (both 
stained  and  unstained).  Examine  the  liquid  in  the  other  tubes  in  the  same 
way.  Remove  a  little  of  the  putrefying  mass  from  one  tube  and  dilute  by 
placing  it  in  a  water  blank.  Transfer  a  platinum  loopful  of  this  dilution 
to  a  melted  gelatin  tube  and  another  to  a  melted  agar  tube.  Mix  by  gentle 
agitation  and  pour  into  Petri  dishes.  Allow  to  grow  for  two  days  and  ex- 
amine the  colonies.     Are  both  liquefiers  and  non-liquefiers  present? 

No.  22.  Ammoniacal  Fermentation  of  Urea. — Fill  a  test-tube  or  a  flask 
half-full  of  urine  and  allow  to  stand  for  a  day  or  two  in  a  warm  place. 
Note  the  odor  of  ammonia.  Suspend  a  bit  of  red  litmus-paper*  in  the 
mouth  of  the  tube  and  note  that  it  turns  blue  from  the  ammonia  fumes. 
Remove  a  bit  of  the  liquid  with  a  platinum  loop  and  examine  (stained) 
under  microscope.     Note  the  immense  number  of  bacteria. 

No.  23.  Manure  and  Sewage. — Place  a  small  drop  of  sewage  and  a  small 
bit  of  manure  in  separate  water  blanks.  After  thorough  mixing  remove  a 
loopful  in  each  case  and  transfer  to  a  second  water  blank  for  further  dilu- 
tion. After  again  mixing  transfer  a  loopful  of  this  second  dilution  to  melted 
agar  or  melted  gelatin,  gently  agitate  and  then  pour  into  Petri  dishes. 
Allow  to  grow  for  two  or  three  days  and  note  the  number  of  colonies,  in- 
dicating the  great  numbers  of  bacteria  in  the  original  materials.  A 
quantitative  determination  can  be  made  if  desired  by  using  i  c.c.  of  the 
sewage  or  i  gram  of  manure,  and  diluting  100,000  times. 

No.  24.  Soil  Bacteria. — For  general  study  use  standard  media  as  already 
described,  adjusting  the  reaction  to  0.5  per  cent,  acid  instead  of  1.5  per  cent, 
and  using  1.2  per  cent,  agar  and  12  per  cent,  gelatin.  Plates  made  with 
these  media  inoculated  with  soil  will  not  fail  to  show  numerous  soil  organisms, 
bacteria  and  molds  being  very  abundant.     To  obtain  proper  samples  of 

*  Filter-paper  moistened  with  Nessler's  solution  (used  by  chemists)  is  better,  which  should 
turn  yellow  to  reddish-brown  if  ammonia  fumes  arise. 


320  LABORATORY    WORK. 

soil  for  the  study  of  various  soil  problems  requires  special  methods  and  spe- 
cial care  too  difficult  for  elementary  work.  The  relative  number  of  bac- 
teria in  different  soils  may  be  determined  as  follows:  Select  two  soils  for 
study,  preferably  one  rather  sandy  and  the  other  filled  with  humus.  Ob- 
tain a  sample  by  mixing  the  soil  well  with  a  spade  and  take  to  the  labo- 
ratory about  loo  grams.  Mix  thoroughly  the  sample  and  weigh  out  one 
gram.  It  is  best  to  do  this  after  passing  the  soil  through  a  sieve.  Place 
in  a  99  c.c.  water  blank  and  shake  vigorously  for  two  minutes.  Transfer 
I  c.c.  of  this  to  a  second  99  c.c.  water  blank  and  mix  well.  Transfer  i  c.c. 
to  a  9  c.c.  water  blank  and  mix  again.  From  this  transfer  i  c.c.  to  a  Petri 
dish  and  then  pour  upon  it  the  contents  of  a  melted  agar  or  gelatin  tube. 
Mix  in  the  usual  way,  harden,  and  after  two  to  four  days  count  the  number 
of  colonies  in  the  two  soils.  This  will  give  the  relative  numbers  approxi- 
mately only.  To  obtain  them  exactly  allowance  must  be  made  for  the 
water  in  the  two  soils. 

No.  25.  Denitrifying  Bacteria. — Make  a  broth  containing  1000  c.c. 
water,  i  gm.  peptone  and  2  gm.  potassium  nitrate.  Fill  a  few  fermentation 
tubes  and  sterilize  by  steam.  Inoculate  several  with  a  little  soil  from  differ- 
ent localities.  Incubate  at  ordinary  room  temperature  for  several  days. 
Gas  will  appear  in  the  closed  arm  if  denitrifiers  are  present.  This  gas  is 
mostly  nitrogen  and  represents  so  much  loss  to  the  soil.  The  bacteria  can 
be  isolated  by  the  plate  method  if  desired. 

No.  26.  Nitrogen  Fixers. — Their  action  may  be  shown  as  follows: 
Make  a  solution  containing:  MgSO^  2  gm.,  KjHPO^  2  gm.,  CaCl  0.2  gm., 
dextrose  2  gm.,  citric  acid  5  gm.,  FeC\^  traces,  water,  (distilled),  1000  c.c. 
Make  the  reaction  neutral,  taking  great  care  not  to  pass  beyond  the  neu- 
tral point.  Place  some  of  this  in  a  flask,  sterilize  by  steam,  and  inoculate 
with  a  little  soil.  Incubate  at  ordinary  room  temperature.  After  two  or 
three  weeks,  growth  will  be  evident  and  usually  a  membrane  appears  on 
the  surface.  This  membrane  contains  nitrogenous  matter  and  since  there 
is  no  nitrogen  in  the  culture  medium  as  above  made,  the  nitrogen  must 
have  been  assimilated  from  the  air.  The  isolation  of  these  bacteria  is 
very  difficult. 

The  study  of  the  nitrifiers  is  too  difficult  to  be  undertaken  by  elemen- 
tary students. 

No.  27.  The  Presumptive  Test  for  Bacillus  coli. — This  test  is  frequently 
made  to  determine  whether  water  is  suspicious.  Fill  fermentation  tubes 
with  lactose  bouillon  and  inoculate  five  tubes  with  i  /2  c.c.  and  five  more 
with  I  /lo  c.c.  of  the  water  to  be  tested.  Place  in  the  incubating  oven  for 
twenty-four  hours.  If  gas  appears  in  the  closed  arm  sufficient  to  fill  it 
from  I /id  to  1/4  full,  the  test  is  positive  and  the  water  probably  contains 
B.  coli.  To  determine  this  absolutely  requires  further  tests  that  cannot 
be  given  here.  The  purpose  of  using  the  different  amounts  of  water  is  to 
give  a  rough  idea  of  numbers.  If  gas  appears  in  all  of  the  i  /lo  c.c.  tubes 
it  will  indicate  that  there  are  probably  more  than  10  of  the  gas-producing 
organisms  per  c.c.  of  the  water.  If  it  appears  in  the  1 1 2  c.c.  tubes  but  not 
in  the  i/io  it  indicates  that  there  are  less  than  10  per  c.c.     This  pre- 


MATERIALS.  32 1 

sumptive  test  is  only  of  value  in  suggesting  suspicion,  but  insufficient  to 
state  the  presence  of  sewage  contamination. 

No.  28.  Bacteria  from  the  Root  Tubercles  of  Legumes. — -The  bacteria 
that  have  been  commonly  supposed  to  cause  root  tubercles  may  be  obtained 
as  follows: 

Add  5  gm.  of  wood  ashes  to  1,000  c.c.  water,  heat  in  steam,  boil  for 
one  minute  and  filter.  To  the  filtrate  add  10  agar  and  4  grm.  maltose 
(or  some  other  sugar) ;  heat  in  steam  till  dissolved,  boil  one  minute  and 
filter.  Place  in  test-tubes  and  sterilize  by  steaming  on  three  successive 
days,  or  in  an  autoclave  for  one  hour  at  10  pounds  pressure. 

Select  some  vigorously  growing  legume,  not  too  large,  and  with  a 
spade  dig  up  its  mass  of  roots  still  embedded  in  soil.  Wash  away  the  soil 
from  the  roots  and  probably  plenty  of  tubercles  will  be  found  attached. 
Remove  a  small  tubercle  with  forceps,  wash  in  tap  water  and  then  immerse 
in  a  solution  containing  500  c.c.  water,  i  gram  corrosive  sublimate,  and  2.5 
c.c.  HCl.  It  should  be  immersed  in  this  for  two  to  three  minutes  to  steril- 
ize the  surface.  Then  the  tubercle  is  held  between  folds  of  filter-paper  that 
has  been  moistened  in  the  sublimate  solution  and  a  gash  is  cut  in  it  with  a 
hot  knife.  A  platinum  needle  is  then  sterilized  and  some  of  the  central 
mass  of  the  tubercle  removed  and  placed  in  a  drop  of  sterile  water  on  a  slide. 
Place  a  drop  of  sterile  water  in  each  of  several  Petri  dishes  and  transfer  a 
small  loopful  of  the  drop  on  the  slide  containing  the  tubercle  contents,  into 
the  water  drop  in  each  of  the  Petri  dishes.  Pour  into  each  a  tube  of  the 
maltose  agar,  allow  to  harden  and  incubate  at  70°.  The  colonies  will  appear 
in  three  to  four  days  and  may  be  isolated  and  studied  in  the  usual  manner. 

No.  29.  Bacteriological  Analysis  of  Market  Milk. — Collect  in  sterile 
bottles  a  number  of  samples  of  market  milk  from  different  milkmen. 
Keep  cool  with  ice  until  they  are  brought  to  the  laboratory  and  then  plate 
at  once.  Dilute  1,000,  place  i  c.c.  in  each  of  two  or  three  Petri  dishes  and 
add  a  tube  of  ordinary  agar.  Incubate  at  98°  for  twenty-four  hours, 
count  the  bacteria  in  each  plate  and  determine  the  average.  This  is 
the  procedure  commonly  used  in  making  bacteriological  analyses  of  mis- 
cellaneous samples  of  market  milk.  If  there  is  any  reason  for  thinking 
the  milk  is  very  old  a  higher  dilution  is  necessary.  If,  on  the  other  hand,  the 
examination  is  to  be  made  of  milk  known  to  be  good  and  in  cool  weather, 
a  dilution  of  100  is  better. 

No.  30.  Bacteria  in  Sour  Milk. — Allow  some  milk  to  stand  till  sour,  but 
not  curdled.  Dilute  it  100,000  times  (two  99  c.c.  and  one  9  c.c.  water 
blanks)  and  make  plates  in  litmus  gelatin  or  litmus  agar.  Incubate  at 
70°  for  two  or  three  days  and  count  the  number  of  bacteria.  Count  the 
number  of  acid  colonies. 

No.  31.  Bacteria  in  Cream. — Make  plates  as  in  No.  30  frora  fresh  cream, 
diluting  1,000  times  and  using  litmus  gelatin  and  litmus  agar.  Make  other 
plates  from  some  ripened  cream  in  gelatin  and  in  agar  and  diluting  100,000 
times.  Incubate  at  70°  for  three  days  and  compare  the  plates  from  the 
two  lots  of  cream  as  to  number  of  bacteria  and  relative  proportion  of 
acid  bacteria. 


322  LABORATORY    WORK. 

No.  32.  Eflaciency  of  Pasteurization. — Obtain  some  milk  ten  to  fifteen 
hours  old.  Make  three  plates  as  described  in  No.  30.  Place  the  milk  in 
a  jar,  and  heat  in  water  to  140°,  keeping  it  at  that  temperature  for  one- 
half  hour.  Make  from  it  a  second  series  of  plates.  Incubate  plates  at  98°  for 
twenty-four  hours.     Count  the  number  of  colonies  in  the  two  sets  of  plates. 

No.  33.  Bacteria  on  Hay. — Place  a  handful  of  hay  in  water  and  warm 
slightly  for  ten  minutes.  The  water  should  be  hardly  more  than  luke 
warm.  This  is  called  a  hay  infusion.  Put  a  platinum  loop  of  the  infusion 
into  a  water  blank  and  inoculate  a  loopful  of  this  dilution  into  an  agar 
plate.      Incubate  at  98°  for  one  day  and  count  the  bacteria. 

No.  34.  Study  of  Common  Molds. — Place  under  a  bell  glass,  or  in  some 
other  closed  box  which  will  prevent  evaporation,  some  pieces  of  bread, 
slightly  moistened,  two  or  three  pieces  of  cheese,  and  some  slices  of  a  lemon. 
Keep  these  in  a  warm  place  for  a  few  days  until  molds  make  their  appear- 
ance. Examine  day  by  day  until  they  become  covered  with  spore  masses. 
Note  the  mycelium,  its  fineness  of  texture  and  the  color  of  the  spore  masses. 
Determine,  if  possible,  whether  the  mold  masses  on  the  different  objects 
are  the  same  or  different  species.  Remove  a  bit  of  the  mycelium,  place 
in  a  drop  of  water  on  a  slide  under  a  cover-glass  and  examine  under  the 
microscope.  Sketch  the  threads  and  their  method  of  branching.  Place 
some  of  the  spores  under  the  microscope.  Sketch.  How  do  they  com- 
pare in  size  with  bacteria  and  yeasts? 

No.  3  5.  Germination  of  Mold  Spoxes. — Melt  two  or  three  agar  tubes  and 
two  gelatin  tubes  and  add  a  few  drops  of  HCl,  just  sufficient  to  make  the 
medium  slightly  acid.  Pour  out  in  Petri  dishes  and  allow  to  harden. 
With  a  platinum  needle  transfer  the  smallest  possible  quantity  of  spores 
from  one  of  the  molds  and  touch  the  surface  of  the  agar  and  gelatin  in 
several  places  with  the  tip  of  the  needle.  Examine  with  low-power  micro- 
scope and  note  that  spores  have  been  planted  on  the  surface.  Set  in  a  warm 
place  and  examine  in  twenty-four  hours  to  see  if  the  spores  are  germi- 
nating. Sketch  a  germinating  spore.  Allow  to  grow  till  spores  are  formed, 
studying  daily  with  microscope. 

No.  36.  Yeast  and  Fermentation. — Grind  up  a  few  apples  in  a  meat- 
cutter  squeeze  the  juice  through  cheese-cloth  and  then  filter  through  filter- 
paper.  Fill  six  fermentation  tubes  with  the  juice,  filling  the  closed  arm 
full  and  the  bulb  half -full ;  plug  with  cotton.  Set  two  of  them  in  a  warm 
place.  Sterilize  the  other  four  by  steaming  for  half  an  hour.  After  steril- 
izing inoculate,  two  with  a  little  yeast  (from  a  yeast  cake)  and  set  in  a 
warm  place.  Examine  in  eight  to  twelve  hours.  Note  the  gas  bubbles 
rising  in  the  closed  arm.  Remove  a  little  of  the  sediment  and  examine 
under  the  microscope  (both  stained  and  unstained).  Note  the  clusters  of 
budding  yeast  cells.  How  do  they  compare  with  the  cells  in  the  yeast 
cake?  After  the  arm  is  about  half-full  of  gas,  test  with  NaOH  as  described 
in  No.  18.  By  the  amount  of  gas  dissolved  by  the  NaOH  determine  how 
much  of  the  gas  is  COj  Does  any  fermentation  occur  in  the  sterilized 
tubes?  In  the  original  unsterilized  and  uninoculated  tubes?  If  fermenta- 
tion occurs  in  the  latter  examine  with  a  microscope  to  see  if  yeast  is  present. 


DISINFECTION.  323 

No.  37.  Decay  of  Fruit. — Select  some  sound  apples  and  inoculate  them 
with  mold  spores  by  dipping  the  tip  of  a  knife-blade  into  a  mass  of  mold 
spores  of  the  growths  in  No.  34  and  thrusting  the  tip  through  the  skin  of 
the  apple.  It  will  be  best  to  inoculate  different  apples  with  each  of  the 
kinds  of  mold  that  have  grown  on  the  objects  in  No.  34,  some  of  which 
will  probably  be  the  species  to  produce  decay.  Place  the  apples  in  a  fruit 
jar,  close  the  mouth,  not  too  tightly,  and  set  aside  in  a  warm  place.  Ex- 
amine day  by  day  and  note  that  decay  soon  begins,  starting  at  the  inocu- 
lated points.  Allow  the  decay  to  continue  till  the  mold  breaks  through 
the  surface  and  produces  spores.  If  decay  does  not  occur,  it  means  that 
the  species  of  mold  used  were  not  those  that  produce  decay,  and  the 
experiment  should  be  repeated  with  molds  from  other  sources. 

DISINFECTION. 

No.  38.  Disinfectants. — Of  the  many  disinfectants  in  use  three  are  of 
particular  practical  value.  These,  in  the  strength  commonly  used,  are  the 
following : 

Carbolic  Acid  Solution,  1-20. 

Carbolic  acid  crystals,  2  5  grams 

Water,  500  c.c. 

In  weighing  the  crystals  of  carbolic  care  should  be  taken  not  to  touch 
them  with  the  fingers  since  they  will  burn  the  skin. 

Corrosive  Sublimate  Solution,  i-iooo. 

Corrosive  sublimate,  i  gram 

Water,  1000  c.c. 

In  making  and  handling  these  solutions  it  should  be  borne  in  mind  that 
they  are  very  poisonous. 


Chlorid  of  Lime. 

Fresh  chlorid  of  lime, 

2  5  grams 

Water, 

500  c.c. 

This  disinfectant  is  cheap  and  effective  if  fresh,  but  it  will  not  keep, 
and  should  be  made  up  at  the  time  of  using.  One  pound  of  the  chlorid 
of  lime  in  six  gallons  of  water  will  make  up  an  efficient,  cheap  disinfectant 
for  disinfecting  walls  and  floors  of  rooms  and  has  the  advantage  of  being 
non-poisonous. 

No.  39.  Testing  Disinfectants. — Mix  the  white  of  an  egg  with  ten  times 
its  bulk  of  water,  and  place  the  material  in  a  series  of  test-tubes  filling  each 
about  one-third  full.  To  the  different  tubes  add  the  following:  a.  no 
addition;  b.  1/4  gram  salt;  c.  i  gram  salt;  d.  2  grams  sugar;  e.  5  grams 
sugar;/,  one  drop  of  corrosive  sublimate  solution  (i-iooo);  g.  six  drops 
of  corrosive  sublimate  solution;  h.  one  drop  of   formalin;   i.  three  drops 


324  LABORATORY    WORK. 

of  formalin;  ;.  one  drop  of  carbolic  acid  solution  (1-20);  k.  four  drops  of 
carbolic  solution;  /.  ten  drops  of  carbolic  solution;  m.  1/8  gram  of  borax ; 
n.  1/4  gram  of  borax.  Place  all  test-tubes  in  the  incubating  oven  and  ex- 
amine at  intervals  to  see  which  of  them  undergo  putrefaction  and  which  are 
thoroughly  disinfected.  Note  how  very  much  more  efficient  some  disin- 
fectants are  than  others.  Which  proves  to  be  the  most  efficient?  It  is 
well  in  this  experiment  to  close  the  tubes  loosely  with  a  cork  to  prevent 
evaporation  of  the  volatile  disinfectants. 

The  Use  of  Disinfectants. — The  ordinary  use  of  disinfectants  is  in  con- 
nection with  disease,  their  purpose  being  to  destroy  disease  germs  and 
thus  to  prevent  the  spreading  of  disease.  They  are  sometimes  used  for 
other  purposes,  such  as  reducing  offensive  odors,  etc.,  but  primarily  they 
are  for  the  checking  of  infection.  There  are  various  methods  of  killing  bac- 
teria which  may  be  applied  under  different  conditions.  Heat,  sunlight, 
drying,  chemicals,  and  disinfecting  gases  are  all  of  use  in  certain  connections. 
The  determination  of  which  is  to  be  used  will  depend  upon  conditions. 
The  first  problem  to  be  settled  in  all  cases  of  disinfection  is  when  and 
where  the  disinfecting  agent  should  be  applied  to  produce  the  desired  re- 
sults.    A  few  practical  suggestions  as  to  methods  may  be  of  value. 

The  Person. — Of  all  sources  of  danger  the  one  of  greatest  importance  is 
the  person ;  first  the  patient,  especially  after  recovery,  when  he  is  to  mingle 
with  other  people,  and  secondly  the  attendants  on  the  patient.  Disin- 
fection of  the  patient  during  the  di'sease  is  rarely  possible,  though  his  skin 
should  be  kept  clean  by  bathing  in  water  to  which  a  little  glycerin  is  added. 
The  nurse,  however,  should  keep  scrupulously  clean.  Her  hands  should 
be  carefully  washed  in  soap  and  water  followed  by  strong  alcohol,  or  the 
corrosive  sublimate  solution  above  described.  Such  cleaning  should  follow 
every  time  that  the  nurse  handles  the  patient  or  any  article  of  clothing  or 
eating  utensils  touched  by  the  patient.  Other  parts  of  the  body  also  need 
attention,  but  not  so  frequently.  The  hair  should  be  kept  in  a  cap  to  pre- 
vent its  getting  contaminated,  for  it  is  difficult  to  clean  and  almost  impos- 
sible to  sterilize  it.  When  the  patient  has  recovered  so  as  to  leave  quaran- 
tine he  should  receive  the  same  treatment. 

Carbolic  acid  solution  is  especially' useful  as  a  skin  wash,  and  is  extremely 
valuable  in  cases  of  cuts  or  skin  abrasions.  If  all  cuts  and  bruises  be 
washed  at  once  in  the  carbolic  solution  (1/20),  many  a  serious  sore  and 
many  a  case  of  blood  poisoning  will  be  prevented.  Every  household 
should  have  a  carbolic  acid  solution  on  hand  for  such  purposes. 

Clothing,  Bedding,  Etc. — These  articles  offer  a  difficult  problem.  The 
following  general  directions  may  be  given. 

Burn  everything  which  is  of  no  great  value. 

Boil  in  water  all  articles  that  can  be  so  treated.  The  boiling  should 
continue  for  half  an  hour  and  will  be  sufficient  for  complete  disinfection. 
Steaming  is  sometimes  employed  for  articles  too  heavy  for  boiling,  such  as 
mattresses  and  carpets.  This  requires  special  apparatus  and  can  rarely  be 
performed  at  home.  Any  article  that  can  be  soaked  in  water  may  be  dis- 
infected by  soaking  it  three  or  four  hours  in  water  containing  one  part  for- 


DISINFECTION.  325 

malin  to  5,000  parts  of  water.  Exposure  to  air  and  sunlight  are  good  dis- 
infectants for  light  clothing,  but  not  for  heavy  articles  like  mattresses. 
There  is  no  good  method  for  their  disinfection.  Where  the  infection  of 
such  articles  is  considerable  the  only  safe  thing  to  do  is  to  destroy  them. 
Excreta. — The  feces,  the  urine,  and  all  discharges  from  patients  are 
most  likely  to  contain  infectious  organisms  and  must  be  handled  and 
treated  with  great  care.  One  of  the  best  methods  of  treating  is  to  place" 
the  excreta  in  a  vessel  and  cover  completely  with  a  chlorid  of  lime  solu- 
tion prepared  as  above  described.  This  should  be  allowed  to  act  at  least 
an  hour  before  the  mixture  is  thrown  into  the  sewer  or  otherwise  disposed 
of.  Ordinary  milk  of  lime  or  dry  slacked  lime  is  useful  in  earth  closets 
or  privies. 

The  Sick  Room. — While  a  room  is  bccupied  by  a  patient  little  can  be 
done  to  disinfect  it.  In  case  of  a  contagious  disease  it  is  desirable  that  cur- 
tains, hangings  and  carpets  should  be  dispensed  with,  since  these  catch 
and  hold  dust.  Little  else  can  be  done  beyond  care  in  keeping  the  room 
clean.  After  the  room  is  vacated  it  is  frequently  desirable  to  disinfect  it  be- 
fore it  is  again  occupied.  Carpets,  curtains,  and  bedding  should  be  removed 
and  disinfected  as  above  suggested.  All  surfaces  in  the  room,  including 
walls,  ceilings,  floors,  tables,  chairs,  and  especially  cracks  around  mop- 
boards  and  the  floor  should  be  washed  freely  with  a  disinfectant.  Corrosive 
sublimate  solution  is  frequently  used,  or  the  chlorid  of  lime  solution.  If 
care  is  taken  to  wash  all  surfaces  thoroughly,  putting  plenty  of  the  disin- 
fectant into  the  cracks,  the  disinfection  is  complete  and  satisfactory. 
Since  this  plan  of  washing  is  rather  long  and  troublesome,  a  simpler  plan 
has  been  widely  adopted  in  recent  years  of  using  a  gaseous  disinfectant. 
The  gas  most  commonly  used  is  formaldehyd  gas.  This  is  applied  in 
various  ways,  but  the  simplest  is  as  follows:  All  cracks  in  the  room  are 
first  sealed  by  pasting  gummed  paper  over  them,  this  including  cracks 
around  mop  boards,  chimneys,  as  well  as  fire-places  and  key-holes. 
Then  a  pailful  of  steaming  water  is  placed  in  the  room  to  give  moist- 
ure. Lastly,  one  or  more  "formalin  candles"  are  lighted.  These  are 
mixtures  of  solidified  formalin  which  give  off  the  desired  gas  when  heated 
and  in  the  candles  there  is  added  a  certain  amount  of  paraffin  that  will 
burn.  These  candles  can  be  obtained  from  any  drug  store  and  upon 
the  wrapper  there  is  always  stated  the  number  of  cubic  feet  that  the 
candle  is  supposed  to  disinfect.  The  number  of  these  candles  to  be  burned 
must  therefore  be  determined  by  estimating  the  space  in  the  room  and 
using  candles  accordingly.  It  as  best  to  use  about  half  as  much  again  of 
the  formalin  candles  as  recommended  on  the  wrapper,  since  they  will 
rarely  be  as  efficient  as  is  claimed  for  them.  After  lighting  the  candles  leave 
the  room  quickly  and  seal  the  door  on  the  outside  with  gummed  paper. 
Leave  closed  for  about  twelve  hours  and  then  open  windows  and  doors  so  as 
to  allow  the  gas  to  pass  from  the  room.  If  a  sufficient  amount  of  formalin 
is  used  this  disinfection  is  thorough. 

The  Stable. — The  disinfection  of  the  stable  is  difficult  because  of  the 
roughness  of  the  lumber  with  which  the  stable  is  made.     A  satisfactory 


326  LABORATORY    WORK. 

method  of  disinfection  is  as  follows:  Remove  all  dirt  from  all  surfaces  in 
the  stable.  This  must  be  done  thoroughly  or  the  disinfection  will  not  be 
complete.  Water  must  be  used  freely  to  moisten  up  the  dry  filth  that 
has  accumulated  in  various  parts  of  the  stable.  The  removal  of  the  dirt 
is  thus  facilitated,  and  the  cleansing  must  be  thorough.  After  such 
cleaning,  the  whole  stable  should  be  washed  with  a  solution  of  corrosive 
'sublimate,  above  given  (i-iooo).  This  may  be  done  by  simply  washing 
with  a  broom,  or  better,  by  spraying,  provided  a  proper  spraying  apparatus 
be  at  hand.  It  must  be  remembered,  however,  that  corrosive  sublimate 
corrodes  metals  badly,  and  no  metal  spraying  apparatus  can  be.  used. 
After  the  thorough  wetting  down  of  all  surfaces  of  the  stable  by  the  disin- 
fectant the  stable  must  again  be  washed  with  water  to  remove  the  disin- 
fectant. Instead  of  corrosive  sublimate,  a  solution  of  chlorid  of  lime  may 
be  used  in  the  same  way  in  washing  the  walls  and  floors.  A  disinfection 
of  a  stable  with  formaldehyd  or  any  other  gaseous  disinfectant  is  impos- 
sible, since  the  stables  are  never  tight  enough  to  prevent  the  gas  from  escap- 
ing rapidly. 

The  Dairy. — The  disinfection  of  the  dairy  must  follow  along  essentially 
the  same  lines  as  the  stable.  Everything  must  first  be  cleaned  as  thoroughly 
as  possible,  and  then  all  woodwork  may  be  washed  with  corrosive  sub- 
limate, or  better,  with  a  3  to  5  per  cent,  solution  of  carbolic  acid. 
These  solutions  must  not  be  used  for  washing  the  vessels  which  contain 
milk.  For  cleaning  these  vessels  nothing  but  boiling  hot  water  and  steam 
are  legitimate.  After  the  disinfection  of  all  parts,  the  whole  must  be 
washed  with  water. 

Other  localities  inhabited  by  animals.  To  disinfect  the  barn-yard 
in  which  cattle  are  allowed  to  roam  is  practically  an  impossibility,  and  the 
same  thing  is  true  of  the  pig  pen.  The  amount  of  moist  material  accumu- 
lated in  these  localities  is  so  great  as  to  make  disinfection  impractical  by 
any  means  yet  devised.  We  must  make  the  same  statement  in  regard  to 
pastures  where  infected  cattle  are  allowed  to  roam.  To  disinfect  a  pas- 
ture is  an  impossibility;  it  must  be  left  to  the  action  of  sunlight  and  rains, 
and  these  will,  in  the  course  of  time,  commonly  produce  the  disinfection. 


INDEX 


Abortion,  288 

Abscesses,  289 

Acetic  fermentation,  30 

Acidity,  determination  of,  309 

Acidity  of  soil,  120 

Acid  liquefiers,  152,  198 

Acquired  resistance,  257 

Actinomyces,  12,  13 

Aerobic  bacteria,  19 

Agar  culture   medium,    preparation 
of,  308 

Air,  bacteria  in,  313 

Air,  relation  to,  19 

Alcoholic   fermentation,    2,    25,    42, 
212,  322 
of  milk,  157 

Alinit,  119 

Ammoniacal  fermentation,  53,  319 

Amyolytic  fermentation,  25,  27 

Anabolism,  23 

Anaerobic  bacteria,  19 

Analytical  processes,  24 

Animalculae,  2 

Anthracnose,  295 

Anthrax,  258,  280 

method  of  infection,  282 
preventive  inoculation,  284 

Antiseptics,  243 

Ascococcus,  12 

Aspergillus,  7 

Autoclav,  18 

Azolitmin,  316 

Azotobacter  agilis,  94 

Bacillus,  12,  13 
alvei,  289 

anthracis  symptomatici,   287 
atrosepticus,  302 
betae,  305 

Bulgaricus,  148,  159 
carotivorus,  305 
choleras  suis,  286 
cloacae,  303 
coli,  130,  148,  320 
cubonianus,  304 
cyanogenes,  1 56 
danicus,  94 
denitrificans,  67 
erythrogenes,  156 


Bacilus,  fluorescens,  52 

lacto  rubifaciens,  156 

lactis  viscosus,  1 54 

larvae,  289 

mallei,  286 

mycoides,  55 

necrophorus,  289 

olericeae,  305 

pleurosepticus,  285 

prodigiosus,  156 

radicicola,  98,  99,  100 

solanacearum,  302 

savastanoi,  305 

solanisaprus,  302 

stutzeri,  55 

subtilis,  52 

suipestifer,  285 

tetanus,  287 

tobacci,  226 

tracheiphilus,  303 

tuberculosis,  261 

tumifaciens,  305 

Zeae,  303 
Bacillus  carriers,  162 
Bacteria,  meaning  of  term,  9 
Bacterial  treatment  of  sewage,  81 
Bacterioses,  301,  304 
Bacterium,  12,  13 
Bact.  aceti,  218 

acidi  lactici,  145,  183 

aerogenes,  147,  202 

anthracis,  280 

hyacinthi,  304 

malveacarum,  304 

phaseoli,  304 
Bedding,  disinfection  of,  324 
Beggiatoa,  116 
Bird  pest,  291 
Bitter  butter,  184 

cheese,  202 

milk,  155 
Black  leg,  287 

rot,  298 

spot  in  cheese,  203 
Blight,  295 
Brie  cheese,  204 
Blue  milk,  i  56 
Blue  spot  in  cheese,  203 
Boiling  of  milk,  172 


327 


328 


INDEX 


Bouillon,  preparation  of,  311 

Bread  raising,  214 

Brown  hay,  241 

Budding,  8 

Burnt  hay,  241 

Butter,  bacteria  in,  192 

By-products,  25 

Camembert  cheese,  204 
Canning,  245 

Carbolic  acid,  solution,  323 
Carbon  cycle,  45 

transformation  of,  41 
Cattle  plague,  291 
Cellulose,  43 
Cheddar  cheese,  197 
Cheese  ripening,  195 
Chlorid  of  lime,  323 
Cholera,  Asiatic,  129 
Cider,  214 
Cisterns,  133 
Cladothrix,  12 

Clostridium  Pasteurianum,  93 
Clothing,  disinfection  of,  324. 
Cocci,  10 
Coccus,  13 
Cold  as  a  preservative,  242 

relation  to,  18 
Colonies,  9 
Combustion,  42 
Commercial  fertilizers,  122 
Compost  heap,  35,  77 
Concentrated  milk,  176 
Condensed  milk,  176 
Contact  bed,  84 
Cooling  of  milk,  169 
Corn  wilt,  303 

Corrosive  sublimate  solution,  323 
Cow,  care  of,  165 
Cream,  bacteria  in,  321 
Cream  ripening,  181 

control  of,  185 
Crown  gall,  305 

Dairy,  disinfection  of,  326 

inspection,  179 
Decay,  26,  33,  50,  51 
Decomposition,  24,  50 
Denitrification,  30-36 
Denitrifying  bacteria,  67,  320 
Diarrheal  diseases,  163 
Digestion  by  bacteria,  151 
Digestion  of  curd,  151,  196 
Dill  pickles,  224 
Diastase,  28,  215 
Diphtheria  in  milk,  163 
Diplococcus,  12 
Discontinuous  heat,  18 


Disease  germs  in  milk,  161 
Diseases  of  plants,  293 
Disease,  relation  of  bacteria  to,  2 
Disinfectants,  323 
use  of,  324 
Drinking  water,  sources  of,  130 
Drying,  237 

Edam  cheese,  197,  203 
Eggplant  rot,  302 
Eggs,  preservation  of,  248 
Excreta,  disinfection  of,  325 
Excretions  of  bacteria,  24 
Enzymes,  29,  30,  226,  231 

list  of,  30 

secreted  by  bacteria,  33 

Fallowing,  122 

Farcy,  286 

Farm  Sewage,  87 

Faults  in  cheese,  201,  208 

Fermentation,  2,  25,  29,  35,  36 

tubes,  317 
Filter  bed,  84 
Filtered  water,  133 
Filtering  milk,  170 
Fire  blight,  304 
Fission  fungi,  9 
Flagella,  11 

Flavors  in  cheese,  196,  199 
Flesh,  preservation  of,  238 
Food  of  bacteria,  20 

supply,  37 
Foods  of  plants,  38 
Foot  and  mouth  disease,  290 
Food  rot,  289 
Fore  milk,  139,  169 
Form  of  bacteria,  10 
Foul  brodd,  289 
Fowl  cholera,  285 
Fruit,  decay  of,  323 

preservation  of,  239 
Fruity  cheese,  202 
Fuchsin  solution,  314 
Fungi,  5,  6 
Fungoid  diseases  of  plants,  293 

list  of,  296 

methods  of  combating,  295 

Galactase,  138,  196 
Garget,  153,  289 

Gelatin    culture    medium,    prepara- 
tion, 316 
plates,  316 
Germicidal  power,  1 59 
Glanders,  286 
Goiddu,  158 
Gorgonzola  cheese,  206 


INDEX 


329 


Gram  stain,  315 
Granville  wilt,  303 
Green  manuring,  108,  123 

Hard  cheese,  197 
Hay,  bacteria  in,  322 
curing  of,  239 
Hemp,  234 
Higher  fungi,  6 
Hoof  rot,  289 
Hog  cholera,  285 
Horse  sickness,  291 
Humus,  40 

Ice,  134 
Indol,  test,  318 
Inflammation,  289 
Inoculation,  protective,  258 

diseases  controlled  by,  260 
Inorganic  foods,  39 
Iron,  117 

salts,  38 
Immunity,  253,  257 
Isolation  of  bacteria,  313 

Katabolism,  24 
Kefir,  158 
Kummys,  157 

Lactic  acid  bacteria,  143 

protective  action  of,  160 

Lactic  fermentation,  30,  34 

Leben,  i 58 

Leeuwenhoek,  i 

Legumes,  use  of,  104 
value  of,  95 

Leptothrix,  12 

Limburger  cheese,  207 

Lime,  39,  1 1 1 

Liming  of  soil,  56,  94,  121 

Linen,  234 

Liquefaction  of  gelatin,  151 

Lockjaw,  287 

Lophotrichic  bacilli,  11-12 

Lumpy  jaw,  288 

Magnesia,  39 
Malignant  tumor,  288 
Manure,  120,  122 

bacteria  in,  319 
fresh  and  ripened,  75 
heap,  bacteria  in,  70 
composition  of,  69 
fermentation  of,  71,  74 
losses  from,  70 
protection  of,  73 
Mastitis,  289 
Mazoon,  158 


Metabolism,  24 

Metacoccus,  12 

Methylene  blue,  solution,  314 

Micrococcus,  12 

Microorganisms,  meaning  of  term,  5 

Microscopic  study  of  bacteria,  314 

of  yeast,  316 
Microsporon,  12 
Mildews,  295 
Milk,  bacteria  in,  137 

bacteriological     analysis    of, 
321 
Milk  bacteria,  growth  of,  1 59 

sources  of,  138 
Milker,  42,  166 

Milk  faults,  miscellaneous,  1 57 
Milking  machines,  168 

room,  167 
Milk  pail,  168 

standard,  178 

vessels,  141,  167 
Mineral  ingredients  in  soil,  40 
Moisture,  relation  to,  19 
Molds,  6 

study  of,  322 
Monotrichic  bacilli,  11,  12 
Motility,  II,  315 
Mucor,  7 

Multiplication  of  bacteria,  14 
Mushrooms,  6,  44 
Mycoderma,  217 
Navel  ill,  289 
Neutral  types,  1 53 
Nitragin,  107,  119 
Nitrates,  38,  47,  57,  59,  77 
Nitrification,  30,  57,  63,  64,  65 
Nitrifying  organisms,  58,  60,  61 
Nitrites,  57,  59 
Nitrobacter,  60 
Nitrogen  cycle,  89 

fixation,  92,  loi,  320 

fixing  bacteria,  93,  95 
Nitrogenous  food,  47 
Nitrogen,  loss  of,  90 
Nitrosococcus,  60 
Nitrosomonas,  60 

Number  of  bacteria,  determination 
of,  312 

Oidium  lactis,  224,  240 
Oleomargarine,  bacteria  in,  193 
Olive  knot,  305 
Ophidimonas,  116 
Organic  foods,  39 
Organized  ferments,  26,  31 
Oxidizing  fermentation,  30 


Parasites, 


21,  251 


28 


330 


INDEX 


Pasteurization,  173 

efficiency  of,  322 
Penicillium,  7 

Camembertii,  205 

Roquefortii,  206 
Pepsin,  28 

Peptonizing  bacteria,  55,  150 
Percolating  filter,  84 
Peritrichic  bacilli,  1 1 
Phosphates,  38 
Phosphorus,  113 
Pleuropneumonia,  291 
Potash,  38,  115 
Potato  rot,  302 

tubes,  317 
Powdered  milk,  177 
Preservatives,  chemical,  243 

in  milk,  171 
Preservation  of  food,  236 
Presumptive  test  for  B.  coli,  320 
Proteids,  48 

Proteolytic  fermentation,  30 
Proteus  vulgaris,  52 
Pseudomonas,  12 

campestris,  298 

destructans,  305 

fluorescens,  305 

iridis,  305 

prunii,  304 

Stewarti,  303 

vascularis,  303 
Ptomaines,  25 
Pure  cultures,  9 

in  butter-making,  187 

tobarco  curing,  228 

vinegar-making,  221 
Purification  of  cultures,  313 
Pus  in  milk,  138 
Putrefaction,  26,  33,  50,  319 
Putrid  butter,  185 

cheese,  202 


Quarter-evil,  287 

Rabies,  291 

Rancidity  of  butter,  192 
Rauschbrand,  287 
Red  milk,  156 

Rennet-forming  bacteria,  i  50 
Rennin,  33 
Reservoir,  133 

Resistance  against  microorganisms, 
252-257 
method  of  increasing,  2  54 
Rinderpest,  291 
Rinderseuche,  285 
Root  tubercles,  96,  100,  306 


Root  tubercle  bacteria,  98-101,  119, 

321 
Roquefort  cheese,  206 
Rots,  295,  301,  304 
Rotz  bacillus,  286 
Rusts,  295 
Rusty  spot,  203 

Saccharomyges,  8 
Salt  as  a  preservative,  244 
Saltpeter  plantations,  76 
Saprophytes,  21 
Sarcina,  12 
Sauer  kraut,  223 
Scarlet  fever  in  milk,  1 63 
Self -purification  of  soil,  55 
Septic  tank,  83 
Sewage,  bacteria  in,  319 

composition  of,  78 

farming,  79 

treatment  of,  78,  81 
Sheep-pox,  291 

Sick-room,  disinfection  of,  325 
Silage,  229 
Silo,  35 
Slimy  bread,  216 

milk,  153 

whey  in  Edam  cheese,  203 
Smuts,  295 
Soft  cheeses,  203 
Soil,  acidity  of,  120 

aeration  of,  121 

bacteria,  319 

infusion,  97 

inoculation,  106,  119 

microorganisms  in,  39 

nitrogen  in,  65 

origin  of,  39 
Sour  bread,  216 

fodder,  233 

milk,  bacteria  in,  321 
Soured  beans,  224 
Spices  as  a  preservative,  245 
Spirillum,  12 
Spoiling  of  food,  235 
Spores,  7,  I  5 
Springs,  133 
Sprinkling  filter,  84 
Stables,  care  of,  166. 

disinfection  of,  325 
Staining  of  bacteria,  314 
Starches,  43 
Starters  in  butter-making,  186 

in  cheese-making,  200 

preparation  of,   187 

use  of,  189 

value  of,  191 
Sterilization,  17 


INDEX 


33^ 


Sterilization,  of  milk,  i  7 1 
Sterilizing,  311 

soil,  118 
Stilton  cheese,  206 
Streams,  contamination  of,  135 
Streptococci,  138,  289 
Streptococcus,  12 

bacteria  in,  145 
Streptothrix,  12,  13 
Sugar,  42 

as  a  preservative,  244 

bacteria  in,  250 
Sulphates,  38 
Sulphur,  115 
Sweet  cheese,  202 

curdling,  i  50 
Swelled  cheese,  148,  201 
Swine  plague,  285 
Swiss  cheese,  197 
Symptomatic  anthrax,  287 
Synthetic  processes,  23 

Tainted  butter,  184 

Takosis,  288 

Tallowy  butter,  184 

Temperature,  relation  to,  16 

Tetanus,  287 

Texas  fever,  291 

Thermophiles,  16 

Thunder  storms  and  sour  milk,  144 

Toad  stools,  6 

Tobacco-curing,  224 

Tobacco,  diseases  of,  228 

Tomato  rot,  302 

Torula  amari,  155,  202 

Toxins,  252 

Transportation  of  milk,   177 

Trickling  filter,  84 

Tuberculin  test,  271 

Tuberculosis,  261 

animals  subject  to,  263 
bovine  and  human,  264 
combat  against,  269 


Tuberculosis,  distribution  of,  266 
preventive  inoculation,  277 
resistance  against,  263 
treatment  of  in  a  herd,  273 
use  of  flesh,  277 
milk,  278 

Tumor  diseases,  302,  305 

Turnip-tasting  butter,  185 

Turnip  taste  in  milk,  i  57 

Typhoid  in  milk,  131,  162 
fever  in  water,  129 

Udder,  140 

Unfermented  grape  juice,  213 

Urase,  54 

Urea,  49 

Vaccination,  258 

Vinegar  as  a  preservative,  244 

eels,  222 

making,  methods,  216,  219 

organisms,  218 

preservation  of,  222 
Viscogen,  176 

Washing  glassware,  308 

Water,  bacteria  in,  127 

Water  blanks,  312 

Water-bone  diseases,  129 

Water,    sewage    contamination    of, 

128 
Wells,  130 
Wildseuche,  285 
Wilts,  29s,  301,  302 
Wines,  212 
Wood,  44 
Wooden  tongue,  288 

Yeasts,  8,  27,  212,  316 
Yoghourt,  158 

Zymase,  33 


UNIVERSITY    OF    CALIFORNIA 
BRANCH    OF    THE    COLLEGE    OF    AGRICULTURE 


THIS  BOOK  IS  DUE  ON  THE  LAST  BATE 
STAMPED  BELOW 


OCT  2  3   1930 


m' 


l^sJ'. 


5m-8,'26 


ffciiY^fc* 


Qorm  ,  H.  V 


C6 


Agricultiral  bacteri- 


ology, 


OCT  a  3  isso 


mr 


APR  17 


MAY   1      i^^ 


?  .  (5  ^  ^  '41 


42744 

LIBRARY,  BRANCH  OF  THE  COLLEGE  OF  AGRICULTURE 


